If you can, take them off and paint them on a flat surface out of the sun..Either way, sand and clean them first and use a good quality exterior paint.
If you want to leave them on your home,(or take them down), when painting them, go to any hardware, Walmart, etc. and get those cheap, plastic handled applicators with paint pads on them. They are great for getting into small openings etc. . Slide them over the slat once or twice and voila---it's done. Also great for painting the up and down paleings, rails, or whatever they are called, on your front porch or going up and down your stairs. You can do all sides from one place, the paint pad handles are perfectly shaped for the job.