1 - Pressure wash or clean well any surfaces to be painted.
2 - Depending on what method of application you are using, protect for over-spray or spills with drop cloths, tape and plastic.
3 - Apply primer to bare aluminum areas as needed, I've had good results with both latex and oil primer.
4 - Apply two coats of acrylic paint preferably by spraying. I'd add Emulsa-Bond in the first coat for better adhesion.