The main suggestions are paint in thin layers until covered and shave out parts of the eye sockets to keep from scraping the paint. I use an Xacto and sand paper. Be careful and consistent to not make weird notches in the plastic and, of course, not cut yourself.
This photo shows where the ICY needs it most.
It also helps allowing the paint to dry over night. I always get excited and put it in too soon. So, I understand if you don't do this.
Blythe needs her eye socket shaved out a bit for painting or fabric, but I cannot tell you the exact location. I have worked on 3 ICYs in a row so this is fresh in my head.
Wondering about paint? I typically use Folk Art All Purpose Acrylic paint because it is rather diverse in colors and it can be applied consistent and thinly. Sometimes I seal it, sometimes I don't. Just whatever I feel like at the moment.
Same as the previous post- If you use this tutorial or suggestions as your guide for customizing, it is at your own risk, as well as, your doll's risk. These are just tips from my experiences not the dolly bible.