I love making sims that look like real people. My main tip would be to get/find these two specific types of photos of them to use as references:
1. Front-facing portrait with a neutral expression (So no smile. A neutral expression gives you a more accurate look at their facial features.)
2. Side profile of their face.
I find those two types of reference photos to be extremely helpful in trying to create a sim that looks like a real person. Beyond that, just be patient & take your time. It takes a lot of fiddling around with the different parts of CAS to get a sim to resemble someone.
