Things You'll Need
Pin-style trim nail gun
If you have a square, unadorned door frame that looks unfinished as it stands out from the surrounding wall, you can finish it out and soften that line with quarter-round trim. This is trim that's flat on two adjacent sides and curved on the third -- if you look at it from the end, it's shaped like a quarter-slice of pie. Quarter-round is used to finish out other trim. Paint or stain the quarter-round molding before you install it, so you don't have to paint it on the wall.
Measure across the top of the door frame where it juts out from the wall, from corner to corner, with a measuring tape. Mark out the measurement with a pencil on a piece of quarter-round trim. Leave at least 3 or 4 inches of space beyond both marks.
Set the quarter-round trim on the miter saw. Position the trim with one flat side down and the other facing the back "fence" of the saw, so the curved side is facing you. Place one of the pencil marks in front of the saw blade.
Swivel the blade to 45 degrees, pointing outward from the part of the trim you measured. Make the cut.
Slide the opposite end of the quarter-round under the blade, lining up the pencil mark. Swivel the blade in the opposite direction, setting it at 45 degrees. Cut the trim.
Set the quarter-round on top of the door frame. Position the trim so one flat side sits on top of the door frame and the other flat side stands against the wall, with the miter cuts angling out and up from the corners. Secure the molding by shooting pin nails into it every 10 to 12 inches with your trim nail gun.
Cut the two side pieces of quarter-round in the same fashion, but don't miter the ends that will meet the floor. Cut those ends straight across instead.
Install the side quarter-round pieces with their miters on top forming 90-degree corners with the miters on the top piece you already installed. Shoot pin nails into the side trim pieces every 10 to 12 inches, as you did with the top trim.