While evil is certainly a difficult thing to measure, it’s exactly where I think you should begin. How evil is your villain? Liar? Burglar? Murderer? Genocidal maniac? Exactly how much evil is your Big Bad packing? Find out what lines your villain is willing to cross and often the rest will fall into place.
What is your villain trying to accomplish? Are they trying to possess a person, a village, a country, a world, the multiverse… or do they want to see all it all burn? Whatever it is, a villain needs a reason to be operating.
It’s good to know exactly how many screws short of a hardware store your villain is. Madness comes in many flavors, but what you need to know is how your villain’s madness prevents them from dealing with your players.
How much power does your villain have? Are we talking a force of nature, or a fart in the wind? Power, in D&D at least, comes from one of the three M: magic, minions, or might. Determine in what proportions your villain has them and you quickly develop which character class best represents you villain.
Exactly how tactically efficient, proficient, and generally apt your villain is. Are we talking Bobby Fischer or Rocky Balboa? Can your villain exploit the weaknesses of their enemies, or are they too preoccupied as their enemies exploit the weaknesses they themselves possess?