Most of my characters I've made are already based off existing ones (though they're not realistic humans, most of them are highly cartoonish, and they're my characters rather than pop ones). I basically take whatever face I have and tweak it until the features of that Sim match what my general appearance of my characters are supposed to look like. In the Sims 3, all of my characters in that game are based off those in the Sims 4. I think I enjoy making Sims based of other characters than I do making like "normal people" made for the Sims lol.
If there wasn't a character that was based off an existing one, 9 times out of 10, I exaggerated stuff I hate about some decorations in humans (such as fashion) to make the most grotesque person imaginable. I spam tattoos, piercings, high heels, and whatever other stuff I don't like onto a Sim and then give them entirely negative traits. I did this for...like two or three Sims I believe? idk why I like fine-tuning Sims to be composed of only everything I hate, I like trolling myself :p
