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:
Avoiding Trim-induced Headaches
Here are a few tips to help you avoid a few trim hassles:
- Whenever you can, cut with the thick side of the trim against the miter saw fence. You’ll be less likely to tear out the narrow tapered edge that way.
- Cutting right up to the pencil mark almost always leaves pieces too long, so remove the pencil line with the blade. You’ll most likely still have to shave off more.
- Sneak up on cuts by starting long and dipping the saw blade into the wood while you work your way to the cutoff mark.
- Trim out the biggest windows first. That way, you can reuse miscuts for the smaller windows and not run out of material.
- When nailing 3/4-in.-thick trim, use 15-gauge 2-1/2-in. nails for the framing and 18-gauge 2-in. brads for nailing to the jamb.
- To prevent splitting, avoid nailing closer than 2 in. from the ends.