To make transparent paint for polymer clay dolls, premix heat-set paints to get the color of your choice; make it dark because you will be adding the paint to a medium. Then, add a tiny amount of your color to Genesis Thick Medium until you achieve the level of color with the transparency you desire. If you want to use Thinning Medium, be sure to follow the recommended ratio of 60:40 paint to medium (to ensure that it will dry).
Tools: The glass dish in these videos is a candle plate; find them for $1 at a Dollar Tree store. Tiny watercolor mop/wash brushes work well but for really tiny heads, I use a cosmetic sponge cut into pieces. Look for the smallest flexible palette knife in the shape shown in the video.