Whenever I'm trying to do a likeness of someone, I'll bring up their picture on my phone to keep referencing while I work on them in CAS. I make extensive use of the zoom tool on the face because it really allows for some detailed tweaking. There are features that can be manipulated in different ways in both body view and face view, so don't miss those.
Make sure you check your Sim's profile as well as front view. You can make adjustments from the profile as well, especially with the nose. Also, you can click on the forehead in this view and change the whole frontal shape of the face. You can give a Sim a flatter face, longer face, or shorter, or even sort of a "snouted" look that pulls the nose, lips and chin forward. Sometimes changing this area can alter the way the Sim looks from the front and help with accuracy.
Pay attention to how the curser changes. For example, in the eye area, there are many different ways to change the eyes like shape of eyelids, distance between the eyes, size of the eyes, height of the eyes, and size of the irises. Some, like I said, are in body/normal view and some are in zoom/head view.
Sometimes I'll start with a preset face that somewhat resembles the person I'm creating, or I'll randomize Sims until I get a good face/head that I like. On that note, be aware that you can also change the shape and size of the head. You can make it thinner or fatter and then shrink or enlarge it. This can make a difference in creating a likeness. You can also widen or thicken the neck.
Play around with different presets for chin, cheeks, jawline, forehead, nose, etc. I usually click one and if it's worse, I use the "undo" button. I use it a lot.
Noses can be changed a lot as well. Nostrils in width, height and holes, the "bulb" of the nose, and the bridge. Again, from different views.
I also like to switch from a close view to full body view to get a more distant look at their face to see if the resemblance is working. Then I'll zoom again and do more tweaking. It's a process.
So play around in CAS. Even try experimenting with a generic Sim first or a premade, just to get acquainted with all of the little adjusters. There are a lot. Once you get to know them and familiarize with them, you'll get more comfortable. But keep in mind, creating a likeness can take a lot of time and effort, so you'll need patience, depending on how exact you want them to be. But you could also go with the caricature route and exaggerate certain features that stand out on the person's face. The Sims 4's art style is good for that.
Good luck to you, hope this helped! :)
