How to Trim a Steel Door

Measure the upper span of the door opening, from inside the angles on the top corners.

Transfer the measurement to a length of flat trim. Use two pencil marks along the bottom edge of the trim, with the measurement between them.

Lay the trim on your miter saw, so the bottom edge faces the back guard fence of the saw and the front is facing up. Move the piece to one side until one of the marks is under the blade. Swivel the blade 45 degrees outward, so it points away from the measured span of the trim. Make the cut.

Move the trim in the other direction to put the second mark in front of the blade. Swivel it to again point outward, so it's 45 degrees in the opposite direction as before.

Nail the trim along the top of the door opening using a finishing nail gun, putting pairs of nails every foot or so. The trim should cover the edge of the steel casing and hang out over the surrounding wall. Shoot one of each pair of nails as close to the steel casing as possible, while still being on the wall. Shoot the other of each pair of nails on the outer edge of the casing.

Cut the two remaining trim pieces for the two vertical sides with 45 miters on the end that will face up, but straight cuts of 90 degrees at the bottoms ends, where the trim will meet the floor or the top of the base molding (depending on the configuration of the wall).

Secure the two side pieces in the same manner as you did the top span, shooting the nails in pairs, but keeping them off the steel casing.

Caulk all around the trim.