I did batman on 12*12 inch glass with acrylic. The idea to create big and lil intricate work to come across all possible problems.

Here are few things I learnt.

Make sure glass you buy or have  doesn't have shark edges and corners.

For outline, take same paints, or any colour that you didnt prepare.

If you are mixing, it is easy to have container to store them. Use soft hands to dust of or else it will damage the paint.

After completing the layer, allow it to dry properly. See through light to spot any places with less paint.

Use light hand to handle exacto knife if you need to correct the the outline, or add fine details.

You can use acrylic sheet, i dont know why but i like glass. 

Dont forget to enjoy the process :)