Most trim carpenters don't even use a tape to trim windows. It's all done by eye, with a sharp pencil, a miter saw and an 18-gauge nailer. Here's how they do it.
Cut a 45-degree angle on one end of the trim and hold it so the short end of the angle overhangs halfway, or 3/8 in., onto the jamb. Then mark the other end flush with the inside of the jamb. That'll give you a 3/16-in. reveal.
Hold the trim 3/16 in. away from the jamb at both ends and along the base of the trim. Nail the trim to the jamb with 1-in. brads spaced about every 6 in. Nail the thick part of the trim to the framing with 2-in. brads.
Cut a 45-degree miter on one end of the trim board. Adjust the miter as needed for a perfect fit. Then scribe the cut length 3/16 in. past the bottom of the jamb. Nail the trim onto the jamb first and then to the framing, as you did with the top piece.
Glue and pin together the miter from both directions with 1-in. brads. Wipe the glue squeeze-out with a damp rag right away.
Repeat all the same steps on the other side of the window, fitting first the top miter, and then marking and cutting the bottom one. Nail the trim into place.
Cut an overly long piece of trim and cut a miter on one end. Overlap the far end to check the fit. Mark and recut the miter as needed for a perfect fit.
Cut a test miter on the other end and check the fit. Adjust the miter as necessary until you're satisfied with the joint.
With the saw still set for the previous miter, flip the trim over and scribe the length for the end that has that miter. Transfer the mark to the front side and make the cut.
If the drywall projects more than 1/8 in., crush in the drywall with a hammer. Just be sure the crushed area will be covered by trim. In this situation, your miters won't be 45 degrees. You may need to go as low as 44 degrees to get a tight miter.
If the drywall projects past the jamb 1/8 In. or less, and is close to the window jamb, just chamfer the edge with a utility knife. Check to see if you've pared off enough drywall by holding a chunk of trim against the drywall and jamb. If it rocks and won't sit flush against both surfaces, carve out some more.
If the drywall's recessed behind the jamb, don't nail the trim to the framing at first. Only nail it to the jamb and pin the mitered corners together. After the window is trimmed, slide shims behind each nail location to hold out the trim while nailing, then cut off the shims. Caulk the perimeter of the trim to eliminate gaps before painting.
If you have drywall that's "proud" (sticking out past the jamb) or recessed behind the jamb, you have to deal with it before trimming or the trim won't lie flat. Here's what to do:
Here are a few tips to help you avoid a few trim hassles:
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.