So I got this ask in my inbox, and though I was going to answer it in my asks, I think a little, actual post-tutorial would be best. But first, a disclaimer, as I like to cover my bases:
By no means am I an artist; my process is very much a case-by-case basis, and I edit as I need to for each picture that I do end up editing. My method is far from perfect but it is one that works for me for -my character-, so you may have to experiment on your own using these tips to find a method that suits your wants.
This little guide is using ClipStudio Paint, but you can get the same/similar results with really any photo editing program you choose that has brushes (GIMP, PaintTool Sai, Photoshop, Photopea (which is free), etc)
I'm going to use a painted over face texture as an example (basically a face diffuse I just painted over with it's base skintone), versus a full-on gpose shot (this way I can get a good swatch/an approximate measure of what skintone I'm used to working on).
Big Tip: ALWAYS WORK IN LAYERS, this way you can adjust everything accordingly without going back a million steps.
Use a basic soft-spray brush for your base, and pick a color a few shades lighter than the skintone you're working on (use your color-picking tool, it is your best friend). Set your first work layer to "add (glow), and make a little splotch like this. You can either leave it as is or blur it out/adjust the layer opacity if you desire, but again, it all depends on the state of your gpose/where you're adding it. I often blur out the edges to help blend it into the skin more naturally, on a low setting as to not erase it completely.
Before & after adjusting blur/opacity:
Next, you'll want a basic shimmer brush; there are dozens of free ones out there, so look around and experiment! Just like before, you're gonna want to make another layer, but this time, set it to white (or gold/copper if your character has medium/darker skintones, as it flatters them more and doesn't wash them out where as white is more for fair skintones).
Unfortunately, I can't seem to find the source of the shimmer brush I use, but what works for me may not work for you so it would be best to look at what's available and what suits your tastes more.
Now, working in layers using the "add (glow)" effect, slowly build up the shimmer, first, adding a quick couple swipes, then build it up. Once you're satisfied, focus more on the center
TIP: If you want to make it look more natural, when building your layers, blur out the edges a small bit to hide any possible lines and creases.
And that's pretty much it! Adding shimmer effects is all about trial and error, and just simply adding shimmer where your character catches the light.
I'd post examples but a lot of my shimmer examples are on very much lewd gposes and I'm not trying to get tumblr staff mad at me.
And again, another disclaimer: this is not a one-size-fits-all-guide, I seriously cannot emphasize enough that the best thing for you to do is just EXPERIMENT, this is merely a guideline to give you a lil push in the right direction. Editing gposes is supposed to be fun, and the best way to get experience is to simply practice and learn your tools with your own hands.
I hope this helps, and have fun gposing!
