How do you decide what to use in an encounter?

[Khosan]

I want to start DMing for my friends, but I'm having some difficulty with one specific aspect of encounter design. I get the broad strokes with CR vs. party level and trying to keep them interesting with alternative mechanics or secondary objectives, that all makes sense to me and I don't think I'll be at a loss there.

Mostly what I'm struggling with is just dealing with the sheer quantity of potential enemies and trying to whittle down the list to what feels right for the fight. Especially when I'm searching through physical books. I dunno if I'd struggle less with the concept if I had everything digital instead, but it would at least save me a lot of page flipping. Typically, I just end up feeling overwhelmed, even when trying to filter the list down by CR/habitat/etc. It honestly gives me a similar vibe as building characters in 3.5e, just way too many options that I feel like I have to at least skim through because there might be something really good/flavorful/fun in there.

The solution is probably something like just building up a lot of familiarity with monsters and their capabilities. But I would still be interested to see how other people do it.

[Silly Name]

Well, encounter-building doesn't happen in a vacuum. You usually have a scenario in mind, which should inform what types or sorts of monsters should appear or not appear.

To this end, an useful tool before you become familiarised with the Monster Manual is the Appendix B of the Dungeon Master's Guide: Monsters List! The first list, especially, which divides monsters by environment and sorts them by CR can tell you what is worth looking up - if you're trying to design an encounter in the frozen realm of the Ice Giants, you don't have to look at monsters usually inhabiting the desert! And if you have Xanathar's Guide to Everything, the section on random encounters in that book is an useful addendum or even substitute for Appendix B.

Lastly, a bit of advice: if you find that your encounter makes you constantly flip through the MM, just scan or write down the monsters' stats (or at the very least the most "essential" ones: HP, AC, speed, Ability scores, Saving Throws and Attacks), so that you don't have to constantly switch pages.

[PhoenixPhyre]

I think thematically first, starting with the worldbuilding. Knowing what kinds of creatures inhabit that particular area/make up that particular opposed force.

For example, one of my parties just had a set of (avoided) encounters in a ruined city. I happened to know that this city was populated almost entirely by goblins...but feral ones (relative to others). Ok, so that's goblins, but trade the weapons for claws. They do have shamans...what does their religion look like? Oh yeah, it's pretty basic. 3 forces (fire/hunger/fertility), so I'll take a basic NPC druid block and throw on some relevant spells, adapting it for goblin. The goblins only real power is numbers, so these will be swarm encounters (ie ~10-ish enemies, which is about as many as I'm willing to throw on the board at a time).

Note I didn't worry about CR here--the party was level 9. Individually, these encounters would have been fairly easy. But the risk is in making noise and drawing another group and it snowballing.

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Another group (level 11) ran into a band of hostiles "accidentally" (both going in roughly the same direction). I knew that was part of a crazy apocalyptic druid cult that is hiring tribal mercenary support from the surrounding areas (and further away), so they had an archdruid, some fire/earth elementals, and a bunch of berserker NPCs.

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Basically, decide "what does the world look like here", starting with terrain. If you don't have any other ideas, just use D&D Beyond or something similar to filter by terrain and rough CR (knowing that CR doesn't matter all that much in this edition). Then look for inspiration and go from there. And write it down--that way the next encounter in that terrain has a starting point.

Some parings just make thematic sense--goblins and worgs. Others don't--a group of paladins probably isn't traveling with a bunch of skeletons...or maybe there's a reason for it!

And then practice. Knowing that there really isn't a right answer or a wrong answer. As you get more familiar, you'll start having mental short lists.

[Sigreid]

I start with broad stroking the scenario. Is it demon worshipers attacking the spring festival? Goblin bandits set up near the village? Etc. That will lead me to what creatures and traps make sense for the situation.

[Oramac]

Basically, decide "what does the world look like here", starting with terrain.

Pretty much this. Start with the world (terrain, weather, time-of-day, etc.), and go from there. What lives in the area? Why is it attacking the party, or, why is the party attacking it?

Also, what's the theme of the campaign? Try to flavor things for that theme. If you're running a dragon-centric campaign it wouldn't make much sense to have a bunch of Illithids show up to fight (or would it!?).

[Unoriginal]

And if "start with the world" is still too overwhelming an approach, there is the alternative:

"Start with the cool factor".

Flip through your book at random until something catches your eyes. Could be an artwork, could be a name, could be a line in the description or a feature in the statblock. Could even be the creature's STR score, it doesn't matter so long as it catches your eyes.

Then read the whole entry and see if you like it still. If you like it as-is, then BAM! You have your encounter. If you don't like it as-is, change anything you don't like. Maybe that one would be cooler with a Greatsword. Maybe they'd be cooler if adapted for underwater life, or if they had shadow powers. Maybe you don't like them at all after all but the entry made you think of another thing either related to them or not.

If what you find is too strong for the encounter you wanted, consider making them a later boss. If they're too weak, consider adding other beings on their side.

Then, once you have a cool image of the encounter in your head, build around that. Why is the Wererat working with the Illusionist? How did the Hill Giant get plate armor? Why would a Bard and two Boars want to fight the PCs?

[Mastikator]

Worldbuilding first
Then "what is fun to play for me as a DM"
Then "what makes sense from a story perspective"
Then "how can I use the environment/loot to tell the story"

I typically look for types of monster rather than specific monsters, "it would make sense for there to be a spell caster and a fire elemental here" then I look for NPCs with spellcasting and creatures with the elemental type and fire resistance. When I pick something out that looks good I'll read their stat block and try to figure out environments and scenarios where these would thrive or be weak. And try to design the local environment based on that.

[Corpus]

If you played 3.5 you probably read through books casually as a player.

Do the same as a DM, pick up the DMG, MM, and PHB for casual reading and let your imagination run with ideas. Write what comes to mind (i.e. Orcs on elevated terrain, wolves in tree cover). Then draw on those ideas and flesh them out later.

Sitting down to build encounters at times can be overwhelming, especially if it's game night and you will be playing soon.

[No brains]

I'm going to have to go against others who say story/ worldbuilding first. It's a mistake to bind yourself to what's expected of your world. Fantastic stories defy expectations and in your capacity as DM, in charge of all the gods and mortal networks, you have free reign to put whatever you want where you want. In fact, things that stick out as odd can be useful as a story thread. 'What's a desert-dweller doing in the taiga?' 'Who could have bound that spirit here?' 'Why is there a cow in the house?' all those can spur investigation and drive a story.

What you should really think about first is playing to your party's abilities. Going specific is good, but even painting in broad strokes can work. Generally you want everybody to take at least one turn to deal with a threat, so either give them multiple targets or a big pool of hp to attack. If somebody does better damage, give them a big soft target like an ogre they can blast through. If somebody can hit multiple targets, open a barrel of kobolds. You should also be aware of your party's defenses and when to make them a benefit or detriment. If someone with a good con save hasn't had a chance to shine, maybe dropping a Stinking Cloud will give them a chance to act when others couldn't.

This segues into another idea: familiarize yourself with spells as well as, or even instead of, monsters. Knowing what can hit an int or a cha save can make a difference and you can always modify monsters to accommodate spellcasting ability. A 'Kobold Battlemagus' might be able to have the hp to satisfy your blaster while also having the wis-save spell to make the cleric feel smart, all while having a handy excuse to have kobods with them.

[Oramac]

Sitting down to build encounters at times can be overwhelming, especially if it's game night and you will be playing soon.

This is so true!! If I have even the barest inkling that a fight will happen between the party and , I start planning the fight [I]weeks in advance. My group is currently 18th level, and I've been planning the 20th level end-of-campaign fight since they were around 13th or 14th level.

The bonus here is, even if the party doesn't fight said creature, you can always reuse the fight somewhere else, usually with minimal changes.

[Demonslayer666]

I like to use the encounter generators, like donjon or kobold fight club. I generate several until one sparks my interest, then tweak it a bit. I usually keep several in my back pocket in case the party takes an unexpected turn.

[sithlordnergal]

Making things thematic should be first and foremost in your encounter design. PhoenixPhyre has it right on the money: figure out what kinds of creatures inhabit that particular area/make up that particular opposed force.

Are you in a graveyard that's being haunted by the spirits of the dead? There are going to be undead of some kind in it, maybe a few Constructs or Gargoyles that defend a mausoleum, perhaps some things that feed on the dead like Beasts or even an Ooze. You might even find a Necromancer messing around with things. Try to stick to the theme of what you might find in a haunted graveyard. Now, that isn't to say you can't break those themes. But this should be done sparingly, and should really only be done to make a particular area stand out in some way.

Additionally, stick to creature types when you're selecting the monsters for your themes. At least, stick to them until you have a better handle on what's in the Monster Manuel. That way you aren't overwhelmed by all the different creatures you can choose from.

[Derges]

I start by going to an encounter calculator and playing around with some numbers to get a vibe for types of encounters and crs are fair for my party. IE Boss + mooks, a horde of weaklings, equal numbers/anti-party ect.

That leads me to a few combinations, 1 cr x + 2 cr y, 3 cr i + 4 cr j, 1 cr z ect.

Then I look at monsters that are interesting or fitting at those cr's and build little stories around them if required.

[Alexander Atoz]

I've been writing a blog that (among other items) is dealing with this exact issue. I'm providing at least four, and generally 6-8, other monsters that seem to be a good match for each monster in the Monster Manual. If you'd like to use it, go to http://dragonencounters.com.
The monster suggestions are in a spoiler box titles "Minions/allies" at the beginning of each article. I arranged them into combat encounters, but you can use them individually as well, if you prefer.
The only catch is that the blog is still in its early stages. Even so, I have almost sixty monsters covered (from the beginning until ice devil) and I update weekly.

The suggestions given to you are good. I would add, in response to the advice to focus on terrain, that constructs, fiends, and undead are generally universal, not something dependent on terrain. They're also available to any evil villain/mastermind. (There are exceptions, but as a general rule.) The same is also true of the NPC's in appendix B.