Adding window trim is an instant way to bump up the beauty of any room. A miter saw is an essential tool to successfully learn how to install window trim. If drywall is interfering with placing trim against the jambs, you can trim it down with a utility knife. More extensive drywall blockages require removing edges with a hammer. Remove drywall with care so that the drywall will remain intact beyond the outside edge of the trim.

Tips for Success

• Trim largest windows first because scraps might fit on smaller windows.
• Cut directly on your pencil mark for the most accuracy.
• Start on the thickest edge of the trim when cutting.
• Do not nail trim pieces within 2 inches of their ends.

How to Install Window Trim Step by Step

1. Use a miter saw to cut a 45-degree angle on a piece of trim.
2. Hold the cut piece along the top of the window.
3. Position the shorter end of the angled cut so that it’s centered along the edge of the side window jamb.
4. Mark the other end of the trim with a pencil where it meets the inside edge of the side jamb.
5. Cut a 45-degree angle outward from the pencil mark.
6. Hold the top trim piece along the bottom of the top window casing.
7. Nail the trim to the jamb with 1-inch brads every 6 inches.
8. Nail the trim to the framing with 2-inch brads.
9. Select another trim piece and cut a 45-degree angle on it.
10. Hold it against the angled edge of the top trim to see if it fits snugly.
11. Cut again if you need to make adjustments to achieve a perfect corner.
12. Hold the side trim piece in place again and mark it 3/16 of an inch below the bottom of the jamb.
13. Cut the bottom angle.
14. Spread wood glue on the edge of the top miter.
15. Nail side trim to the jamb and frame with 1-inch and 2-inch brads respectively.
16. Nail the mitered corner from the top and side edges with 1-inch brads.
17. Wipe away excess glue.
18. Repeat steps 9 through 16 to trim the other side.
19. For the bottom trim, cut a 45-degree angle on a long piece of window trim.
20. Fit it against one corner of the side trim.
21. If necessary, adjust the cut to ensure a tight fit.
22. To mark the second cut, hold the trim tight against one side and mark where to cut the other side.
23. Make the final 45-degree cut.
24. Spread wood glue on the angled edges of the bottom trim.
25. Fit the trim into place and nail to the jamb and frame.
26. Nail the mitered corners together from side and bottom edges with 1-inch brads.
27. Wipe off any glue that squeezed out of the miters.