Sand the trim and wipe away the dust.
Prime the trim with an exterior primer to seal the wood and protect it from water. Allow the primer to dry completely, and then paint the trim. You will probably need to apply at least two coats of paint to achieve the depth of color desired. Allow the paint to dry completely.
Determine the layout for the trim. It can encase the window with straight cuts at each end, fitting around the window like a box, or the trim can be cut with angled corners so that it fits around the window like a picture frame.
Cut the side trim according to the method of layout you’ve selected. If you’re making straight cuts, cut the side trim so that the cut edges fit even with the top and bottom edges of the window. If you are installing the trim in picture-frame fashion, cut each end of the trim at a 45-degree angle with the inside angle even with the top and bottom of the window. The most efficient way to cut a 45-degree angle is to use a miter saw or a table saw with a miter gauge.
Nail the trim to the wall around the window. The most efficient way to nail trim is to use a pneumatic nailer, which allows one person to handle the job and results in less wood splintering and bent nails.
Cut the top and bottom pieces of the trim according to the method of layout you’ve selected. If you’re making straight cuts, cut the piece to cover the width of the window and the two pieces of side trim. For the picture-frame method, cut the trim the same as the side trim, with the inside edge of the 45-degree angle even with the edge of the window.
Nail the top and bottom trim into place.
Caulk gaps between the wall and trim, and where the trim fits together in the corners. Use a paintable/stainable caulk suitable for exterior use. Touch up the caulk with the paint you used on the trim.