Making Magic Items

[LibraryOgre]

Have guidelines for this been laid out, yet?

[Contrast]

There are some guidelines in Xanathars.

An item of X rarity will take X weeks and X gold to craft and will require a recipe and hunting down creatures of X challenge rating for materials is about as detailed as it gets.

[JackPhoenix]

DMG, p.128, and XGtE, also p.128. At least according to internet, the page information may be off, haven't checked the books.

[Willie the Duck]

What guidelines are you looking for? The DMG technically has guidelines on the process (even if it seems mostly there to discourage people from actually doing so, and you have things that frustrate many like single high-level scrolls being treated like any other extremely rare magic item, and thus effectively almost never worth it). XGtE has fairly thorough rules on the subject (which cuts the time investiture down significantly), but again things are split out by rarity, and that's it.

There are multiple 3PP products out there which try to make 3e/PF/d20-esque formulaic rules for magic item creation, and although I can't speak to any specific product's quality, that is where I would suggest someone go, if they are looking for that level of granularity. WotC has subtly indicated (not through words, but by putting forth two separate options that do not do this) that they don't want to go down that line*, so I highly doubt anything more will be forthcoming.
*Or who knows, maybe the designers would actually love to do this, but all the slings and arrows they've received regarding WBL and the magic item treadmill and so on and so forth have just made them believe that it is not a good financial/fan-happiness move on their part.

[LibraryOgre]

Mostly the basics; I have a player who is REALLY in to crafting, so I want to know the official stuff.

[RickAllison]

Wayfinder’s Guide to Eberron restates the Xanathar’s crafting rules while adding guidelines for creating recipes (called Schema). In a nutshell:

If you have to make one, it’s around half of the time compared to making the item itself, plus you need to have a minimum bonus in a skill (Arcana, Religion, or Nature depending on the creator’s spellcasting type) to make a Schema, ranging from +2 for common to +10 for legendary. An Arcane Trickster, Bard, or Prodigy might be able to make something well out of their weight class, while a Wizard or Artificer will be able to eventually make anything if they get the skill proficiency. For people with sub-18 Intelligence, they still can potentially make Legendary schemes, it just might involve getting a Headband of Intellect first.

Potions and scrolls do not require schema. Potions require herbalism proficiency while scrolls require being able to cast the spell.

The cost in basic supplies is determined by rarity, and you only need gold with specific stuff being possible if you can talk your DM into getting it cheaper. However you also need an item associated with a suitable CR. It could be an actual creature’s part (a lich’s femur for a Wand if Finger of Death, a bear’s claw for Gloves of Primal Savagery), or something you had to conquer or appease to get it like a flower growing in a Sphinx’s lair.

Potions and Scrolls do not have the special item requirement. You just need the money or basic supplies.

At the very least (basic potion of healing made by an Alchemist Artificer), four hours, but it could take a year of work or more for some powerful items.

And once that process is done, you have a magic item!