To install exterior window trim on your home, first sand the trim before priming it to protect it from water damage. Once the primer dries, apply two coats of paint before deciding on a layout for the trim. Cut the side trim, and nail it into place around the window using a pneumatic nail gun. Cut the top and bottom trim, and nail it in place. Caulk the edges, gaps and corners, and then touch up the trim paint.
Some trim designs require straight cuts. Make side trim straight cuts even so that the pieces are flush with the top and bottom trim when pressed against each other. A picture-frame style cut requires each side's end to be cut at 45-degree angles using a mitre saw or a mitre gauge on a table saw.
When cutting the top and bottom pieces, a straight cut piece covers the width of the window plus the two side pieces of trim. A picture-frame cut requires cutting the ends in the opposite 45-degree angle as the side trim so that the pieces fit together to form a rectangle. Choose caulk made for exterior use and that accepts paint or stain. Use wood putty for any nail holes, and paint over the wood putty after it dries.