First of all, a note. Sculpting the face will be the only thing you get done this session. Working on the face requires a lot of fine control, and your fingers will go all over the mini trying to stabilize it. After you've sculpted it, you really don't want to touch it again until it's cured.
With that in mind, let's begin.
Blob some putty on the head, and try to get it as symmetrical as possible on the front half. The back half will be covered by hair later, so it can be rough and somewhat lumpy. Define the jawline early, if there will be issues due to overly thick armature wire or putty cured in the wrong place, you want to know now rather than later. Take some time to smooth everything out, you do not want to be trying to smooth it later while trying to preserve details you actually want. The cup round clay shaper is excellent for this.
Some things really don't change from painting to sculpting. Eye line in the middle of the head, as always. This line will define both the brow and the nose bridge depth, so take your time with placement. Push in at the sides to the outside edge of the eyes, and check to make sure it reads well in profile. You could sculpt a perfect face from one angle, but have it look completely off from the side.
"And now sculpt the rest of the owl!"
Just kidding. Push in the sides of the nose in a sort of isosceles triangle shape, with the upward point at the nose bridge.
"But what if I want a long, narrow, nose?"
Still do this. Creating a vertical strip, and trying to sculpt it into a nose results in a bridge too high and a nose tip too low. The extra putty at the base of the triangle is essential to creating short, flat noses, and thin, large noses.
Also use the flat chisel to push out the bottom of the nose and the start of the mouth. Sculpting a face is like trying to paint with the liquify brush. You want to make as few strokes as possible, before the whole thing devolves into a mess. Multiple, light pushes with the edge of the shaper often gets better results than dragging the shaper through the putty, which often takes some putty along for the ride. Of course, there are a few times that's actually what you want, like the corners of the mouth.
There are some tools you use because they're good almost everywhere, like the flat chisel clay shaper. Then there are those tool you use because they do one thing exceptionally well, like the sculpting needle. Need the tiniest lines possible? Sculpting needle's got you covered. In a sort of overlapping pushing motion, scribe fine lines under the brow to form what will become the eyes. Push with the flat chisel under the mouth line to form the lower lip, and smooth out the outer edges to taste. Pushing with the corner of the clay shaper helps deepen the corners and add some depth, and can be used to add dimples too, if you so desire.
By the way, you're probably not ambidextrous. I'm not. Don't expect symmetry until you've gotten more practice. As you can probably see, I haven't gotten it right either.
Another push on the top of the severe-looking brow forms the top eyelid and lightens her expression somewhat. At this point, the facial features are looking okay... but kind of small and narrow. The ruler also points out something that's been bugging me. A 4mm tall head, with about a 36mm tall figure. That's... 9 heads tall. Ew.
Well, if you can sculpt a smaller face, you can sculpt a bigger one no problem. I was considering scrapping this face and trying again, but decided to see if I could fix it. It is possible to add putty to a face like this, you just have to be very very careful. I plopped a tiny blob on each side of her face, and blended it back in and distributed it a bit more around her face. There, nice, fuller, younger cheeks.
It really doesn't take much putty to fix up proportional errors. We're talking decimal millimeters here. For a 7.5 head figure, that'd be a 4.8mm head, and for an 8 head figure 4.5mm. Not to mention sculpted hair tends to be rather thick, proportionally, and it's much better to sculpt a slightly too small head than a too big one.
On the flipside of that, though, there's "heroic scale", which basically means exaggerated head, and usually hand and weapon sizes. All depends on what you're looking for. Games Workshop humans are something like 6 heads tall. Generally, 7 heads is a good compromise, without being obviously cartoony without being side-by-side with a scale model human.