I always found some creatures in MM weaker than they should be, so I would tweek them to my idea and party strength, no problem. Now my druid wants to use my modified beasts for his wild shape (circle of the moon btw.). I'm okay with that but the problem is I don't know how to determinate final CR of the creature because I've never bothered with that before, I would just change things to make encounter more interesting. Is there a table or something that determinates how much damage/AC/HP should a certain CR have/do, does multiattack alone raise CR or no? There are more things like features that grant adv/pose disadv. but they are homebrew.
Rules for calculating a monster's CR can be found starting on page 273 of the DMG.
Things that affect a creature's CR include:
Damage vulnerabilities, resistances, and immunities