Don't be afraid to start
Painting a keycap for the first time without any prior knowledge can be daunting. As long as you keep your layers thin you can just paint over them. If all else fails you can throw the cap in IPA for some time to strip the paint and primer off. So there is no way of failing. All you need to do after that is re-prime your cap.
For that you can use primers like: Citadel Model Primer
AK Interactive: Fine Resin Primer Spray
when priming make sure to always prime in a sweeping motion across the cap. You want to press down on the rattle can (not pointing at the cap), sweep past the cap, release the spray. Slowly building up the primer.
Shake your paints
Paints (unless you went for tubes) will separate. Before using them make sure to shake them very well. It can be helpful to add 2-3 stainless steel or glass paint agitators to the bottle to make shaking up the paint easier.Thin your paints
Paints right out of the dropper bottle are too thick. If you apply them straight to the keycap they will not only build up texture with brushstrokes but also obscure the details of the cap. You need to add just a touch of water. A good way to do this is using a damp to wet brush and add a little water at a time.Two thin layers instead of one thick layer
Use thin layers to build up color. It might take a little longer but thick layers will mess up the details and will give you an uneven finish. Acrylics are transparent
Acrylics are naturally transparent. Depending on the pigments used some colors are more transparent than others. The yellow pigments used in miniature paints for example always result in quite transparent paints. White on the other hand is fairly opaque.
Don't expect your paint to cover in one layer. An opaque base coat needs about 2-3 layers. The first layers will look uneven. That's okay - just let them dry and add another thin layer.