I liked the way monsters were built in 4e and 13th Age. I'm not sure how to do it in PF2 - I don't recall having seen simple monster creation rules.
Paizo has published monster creation guidelines.
I find them dry and colorless, however. It's all numbers and tables. Things like "a difficult melee brawler at this level should have between 26-29 AC" for example, and then onwards for all the chief stats of the game.
But there is none of the "world building" stuff; the stuff that sets Golarion apart from any random world; nothing of the cool things you see in the monster manuals. Things like how all kobolds (or gnolls etc) share certain traits, strengths and advantages.
What all these tables and procedural steps boils down to is you will get complete but basic monster stats compatible with the bestiary. It's an alternative to doing just that - checking out the bestiary for a comparable beastie (similar level and combat role) and then just stealing its numbers.
I found this
much more inspiring, since the Bestiary doesn't just give you the fundamental numbers; it also provides loads of examples of unique powers, and it showcases which traits and abilities Paizo thinks are suitable for various monsters.
In comparison, the monster creation rules read like having to read through your dishwasher manual... and there aren't any "secrets" to monster creation, or at least, none Paizo shares with us.
Cheers
PS. Neither method is what I'd call "simple". PF2 just isn't a simple game.
Even though the monsters - thank the gods! - doesn't touch player character generation (and its thousands of feats etc), it's still a fairly involved procedure. And you do want to follow the guidelines! For elite creatures leveled higher than the heroes, you need to be somewhat careful; "just winging it" may work in forgiving D&D derivatives, but not here.