Loosen the old window frames using a cat's paw and a hammer to remove the nails.
Use a hammer and pry bar to tap and pry the frame pieces loose (Image 1) and carefully lift out the frames. If the wood surrounding the frames is weak, a temporary support may be needed. Just screw a 2x 2 in the width of the door to the door before you remove the frames (Image 2). This temporary support will need to be removed after you reinstall the new frames.
Carefully remove the glass and set it aside in a secure location.
Flip the doors over to work on the other side.
Note: Garage doors are heavy, so you'll need plenty of help completing this step.
If there's a "lip" around the existing hole for the frame, cut it back flush using a jigsaw (Image 3). Repeat process for each window frame.
Measure the opening and rip wood to the widths you'll need using a table saw.
Also, using the table saw, cut a rabbet in the wood. This is an "L" shaped channel that the glass will rest on.
Using a miter saw, cut the frames to your measured lengths. The style of window frame the homeowners want requires a muntin bar.
Restoration Lexicon: A muntin is a horizontal or vertical bar that divides the sash frame into smaller window lites.To ensure the muntin bar fits flush with the top and bottom frame, recess the ends of the rabbet cut (Image 4). This is not a through cut with the miter saw. Using a chisel and a mallet, remove the indention. Repeat this process for the side jambs of the window frame as well.
To attach the window frames, use a polyurethane glue to hold the joints together. First, lightly dampen the end of each frame and add a small amount of polyurethane glue to each end. Next, connect the frames.
Secure the frames together using finish nails (Image 5).
Lightly sand each frame. An orbital palm sander with 80 to 120 grit sandpaper disks works best.