I haven't found a particularly good way to build sprites either. I fumbled the sprites together with a sprite editor (e.g. GreenFish).
This mod is very much about sprites overlapping each other. This can be done better with layered icons. But it's been a long time and I don't know how to do it anymore.
You may have a look at mod ingteb, (https://github.com/hahoyer/factorio-mod-ingteb/blob/master/core/Graphics.lua). There is some sample code for layered icons, but it is not use at the moment. So expect some difficulties.
