Well, what I would do when spriting a mask is I would start near the eyes. Get the shape of the eyes and then try to do some mask edge and mouth regions. When doing the shading, remember how the light effects the other masks. It tends to shine down from the top left from the observer's point of view. The colors, when you use the pallet, the lightest and darkest colors should be used for outlining 9 times out of 10. There are a few exceptions, but they are few and far between and shouldn't be a factor. But most of all, look at the kit up close. Look at little patterns the kit tends to do based on what kind of shape(s) the object has. Once you get familiar with the kit's trends, then copy them where you see fit (no copy and pasting, that never works unless it's a scope. Then be very careful). And the last little bits I can think of are to look at your reference pic constantly and to zoom in and zoom out often. Follow those and your eye for detail, and youout should be good.