How I prep monsters in 5E D&D (a 7 and a half step process ).
1. I choose the appropriate monster based on the scenario, terrain, my understanding of its relative power throughout my time running D&D of various editions, etc. . .
2. I look up the monster in the 5E Monster Manual or other 5E book that might contain it. I might also google it.
3. I inevitably find it unflavorful and mechanically dull and/or not the power level I want.
4. I look to see if I can find it in MCDM's Flee Mortals
5a. If it is in there, I inevitably quibble with their thematic take but take note of what abilities they give it.
5b. If it isn't there, I look at similar monsters, if present, and take note of their abilities.
6. I look up the monster in the 1E and/or 2E (rarely, 3E) MM and read about it and see how its powers worked (however vaguely written) back then.
7. I make my own version, taking/tweaking powers from Flee Mortals or other sources and/or making them up using the older edition versions as a guide.
7a. If necessary, I peek at
@SlyFlourish and Co's
Forge of Foes and do some
very rough monster math - mostly to have a CR I can use to award XP for defeating it (though this is a new thing, and I am just as likely to just eyeball it).
Basically every time. Today I did it for the Black Pudding. This is not a complaint, tho.
It work for me and I am not sure that even if I found the 5E MM to be a better book, that my inclination to tweak would not lead me down the same basic path. For me, it feels like the culmination of an approach I started doing in 3E (customizing monsters) but not as fiddly.