The best way to paint shutters, both interior and exterior types, is to spray them, using either canned spray paint or an airless power sprayer. But, because that's not always possible, you can still get a quality finish on old shutters by using a brush.
Take the shutters down and scrape, sand, and clean them as needed. Then, if you can hang them from an open ceiling joist -- in the garage, for example -- you can paint both sides at the same time. Otherwise, stand the shutters upright or lay them out on the floor to paint one side at a time.
Keep your brush on the dry side. An excessively wet brush will result in runs and drips and, if the louvers are adjustable, sticking problems. Paint the window side of the shutter first. That way, if you do miss a run, it won't show.
On adjustable shutters, put a wood matchstick or a little wood wedge between the adjusting rod and one or two of its staples to keep the rod away from the louvers. Paint the louvers first with a 1/2- or 1-inch trim brush. Then paint the frame with a 2-inch brush. Leave the shutter edges until last so you can periodically turn the shutter over to check for runs. If you find any, smooth them out with an almost-dry brush before they set. When the front is dry, paint the back.