How to Build Exterior Door Surrounds

Measure the top horizontal span of the doorway, from one inside corner to the one across from it. Mark out the measurement on a piece of door trim, putting two marks on the bottom edge of the board, with the measured span between them.

Set the trim on the miter saw face up, with the bottom along the back "fence" of the saw. Turn the blade so it points 45 degrees out -- to the close end of the trim. Cut it.

Move the trim so that the second mark is under the blade. Turn the blade 45 degrees in the other direction, so it's again pointing to the close end. Make the cut.

Set the trim on the top of the doorway, with the bottom edge lined up with the edge of the casing, and the mitered cuts at the ends rising up and out at the corners. Secure the trim with a finish nail gun, shooting nails every 10 or 12 inches along the top and bottom edges of the piece.

Measure one vertical side of the opening, from the upper corner to the ground. Cut the top end at 45 degrees as before. Cut the other end at 90 degrees, or straight across, so it will sit flat on the floor. Secure the piece with the mitered end butting to the mitered end of the first piece, so it forms the corner.

Repeat the process for the other vertical side of the doorway.

Caulk along the joints where the trim meets the wall and the doorway.