You first outline the design in a black ultra 
marker on the top side, then on the back you put 
the paint in layers, from darkest to lighest.
Any regular art store sells the supplies. Acetate 
comes in a book similar to sketch paper. They 
have different types, so ask the sales people 
about the different qualities. You'll want 
several types of brushes, go for the high 
qualities they work better -- more consistent, 
better application, and have a longer life. 
Aryllic paint can work (it is about the 
cheapest), but they have a large selection of 
possible paints.