It's not as hard as you think! Especially now with all the face items they have. You've got awesome layering skills, and that's really all it takes cat_3nodding
You do it little by little, building up to a full face. Visualize or decide on the kind of expression you want and go from there. If a full face is too intimidating, play around with just a few small things first, like the most basic stuff in steps one through three.
Step one is to pick a mouth that gives the expression you want. It doesn't always have to be an expressive mouth to give the right expression. Sometimes I'm making a character that's giving a dead-pan stare, so the default flat line mouth works for that.
Step two is to put on a nose that doesn't look dumb with the mouth. There are so many great nose choices now luckily. But even just having a nose gives so much more depth and shape already! The Commander noses layer with the Masquerade noses, but mostly noses won't layer together so you gotta just find "the one". The small Lovely Genie nose rings can sometimes look nice, like another nostril, but mostly it looks like a piercing so I usually only use it for that.
Step three is to pick eyes that are a nice shape. For sharp, harsh, or "sexy" expressions, I pick something that's smaller or thinner. For happier or more innocent expressions, I choose bigger eyes. A trick for eyes if you can't find the right ones is to use eye shaping items. For example, Rookie Pursuit and its recolors' His Handsome Gaze, or either Half Lidded Eyes or Judgmental Eyes from The Prince's Rose or The Baron's Rose. Certain makeups will change eye shape too, like the eye shadows from Mysterious Captain and recolors or the Getaway items.
Step four is to make sure your eyebrows are what you want - that is, if the hair you have doesn't have the eyebrows you like, use bangs, Brow Basics, or Bold Brows to get the shape. Eyebrows can go a long way for creating a strong face!
Step five is to layer any makeup/tattoos/scars/wounds you might want to finish off the look if needed. This is a bit optional depending on if you're satisfied with the face already or if the character calls for it. I like to use as little as possible of this step to achieve what I'm looking for so I don't over-do it and look silly. I really like Transgenic Soldier to give shape to the cheeks. There are a few more subtle eye shadows to give eye depth - SDPlus Joey Fuschia or the Lost Boy items, even the Masquerade shadows to a certain extent. Some eyes will already have that built in, like the SDPlus Thorn eyes. Blushes can give cheeks a more life-like feel if you use the right kind/color. I like the Across Blush and Slash Blush personally. Not all makeups layer either with each other or with tattoos/scars/wounds, but quite a lot does, so you can get some really neat effects if you play around!
Anyway, I hope that helps! It took me a long time to figure out face-building and pick out combos I liked, but once you figure it out, it tends to come pretty naturally cat_3nodding