Easy Way to Add Class Levels to Monsters?

[Kinneus]

Did the DMG or MM have rules/guidelines for adding class levels to monsters? Did I miss it somewhere?

I want to make a Hobgoblin Trasmuter as a big bad for a dungeon I'm creating, as well as an Orc with Fighter levels who goes the Battlemaster route. Just wondering the best way to go about this, any advice or input appreciated.

[Balor777]

Deside whats the new challenger rating you want.
Find all monsters with the CR you want.
One of them should look like a good match.
Make SMALL adjustments to the high CR monster and youre ready to go using the CR you choosed with the monster you want to get stronger.

[Dralnu]

DMG has a small blurb about it on p283.

For each class level, the monster gains one HD of its normal type, ignoring the class's HD progression. The monster's proficiency bonus is based on its CR not its class levels. Tweak the CR as you see fit, but realize that some monster/class combinations are more powerful than others and should increase CR more.

Personally, I'm super lazy and just reskin monsters as I see fit. Find a monster that is generally what I want at the right CR, tweak it until it's what I want. I recently made a "hellrider demon" out of a goblin by giving it the demonic template and replacing its armor with natural, scimitar becomes claws, voila.