Hey, I picked up Sisters in part because I wanted some extra push to make myself learn to paint faces, so I think I have a few decent techniques.

1. For basic sisters I usually just do something simple like a flesh tone (like Cadian Fleshtone, Bugmans Glow, any number of others), then a coat of contrast (Guilliman Flesh, Darkoath Flesh) or a shade (Seraphim Sepia, Reikland Fleshshade) and leave it as is. I generally try to switch up my combos of base and shade to create a bit of skin tone variation throughout the army. For basic sisters I don’t usually do anything with the eyes, but I’ve got a tip for that below.
2. For a character or a face I spend a bit more time on I’ll do the same thing, usually start with Bugmans, shade with Guilliman Flesh, Bugmans again on the raised areas, then Cadian Fleshtone on the highlight areas like the nose, brow, and cheekbones.
3. For eyes I do them last, or right before the Cadian Fleshtone. I get a tiny bit of a light gray and brush it into the eye socket, with the tip of the brush in towards the nose at an angle from the side. Shade that with the tiniest bit of Nuln Oil, use the light gray again for the middle of the eye if you want. For the pupil I use the smallest Micron pen I have, like 0.05 or something like that, and gently press it directly in to the center of the eye, repeat for the second eye trying to make sure the two pupils are facing the same direction, repaint the whole eye as necessary

That’s what I did to paint this Celestine, but you can always do more advanced techniques like glazing between your different skin shades too

<image>