Is there some comprehensive guide to creating a homebrew monster? I am running Icewind Dale RotFM and somehow created NPCs, that will be the BBEG after the one in the book, and need to know how to make them balanced for after the campaign. Nothing too crazy, a human pugilist, one-armed dragonborn swordsman, and a human mage. I just don't know where to start honestly, so if there's a guide out there that's like if you give them this, increase their CR by 1 or these spells can be given to someone of CR X or higher, something like that.

There isn’t much, but what there is is in the DMG:

https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonster

I've found this article (and this blog in general) very useful for designing new monsters:

http://blogofholding.com/?p=7338

There are also existing monster statblocks that can be used as bases for NPCs, if you do a search on Humanoids. Some are in the Basic Rules, like Mage, Acolyte and Bandit; and Volo's Guide to Monsters has even more, like Swashbuckler and Enchanter.