get some latex paint and a roller brush thing. Apply a thin coat, wait 15 minutes, and repeat until the layer is thick enough. If you try to apply a thick coat in one go it will actually make it thinner because the paint doesn't stick to the glass very well until partially dry; if you roll over the existing layer before it is dry, the roller will pick up the paint instead of applying it, and if you try and avoid that by using more pressure, the roller won't spin and you will screw up the layer.
In short, apply a thin layer and don't go back over paint you have just applied until it has had time to dry.