Eyes and mouths are a pretty big pain.
What I do, is take the textures of a regular character, like Peter or Adrian Toomes, and cut the mouths or the eyes out of those.
Then, through a lot of trial and error, I cut the mouth roughly in half and find the right position to put it on the mask.
It'll end up looking something like this (just an example):
For the eyes, well, I'll post Bullseye's... eyes, as an example.
This is Spidey's texture in his costume's eyes:
You want to make sure, when placing an eye in there, that the area surrounding the eye is similar to that corresponding area on the mask.
So, if you were say trying to make Iron Fist, you would want the black in the image above to be yellow.
Sorry if I couldn't be any help. I do everything I can to make my skins look good, even if it's a long painstaking process.