How to paint already painted window plantation shutters?

Clean the shutters well, prime with a good quality exterior primer and then put on your final coat. If you tend to dry your brush too much you will get streaks. Leave paint on the brush and slowly move it down over your shutter. If it has horizontal design, then brush that way. You can also use the foam throw away brushes. Same thing applies, not too dry.

Use an HVLP for the easiest and best results. You can buy them inexpensively at Harbor Freight

Use a self-leveling enamel paint to avoid brush strokes. DecoArt makes a waterbased enamel that is fantastic for outside.

Lightly sand to remove any gloss. Prime with Kilz primer then paint your color. Use a foam brush to get a smooth finish. Otherwise spray paint.