Step 1
Measure along one side of the post where it meets the porch. Put the two marks on the back edge of the top side of the molding, with the measured span between them.
Step 2
Set the molding on your miter saw, standing in the position it will be along the bottom of the post. Place the back of the molding against the saw fence. Move the piece so one of the two marks is in front of the blade.
Step 3
Swivel the blade to 45 degrees outward, away from the measured span, and make the cut.
Step 4
Move the piece so the other mark is in front of the blade. Swivel the blade 45 degrees in the opposite direction, so it's again pointing out from the measured span. Make the cut.
Step 5
Set the molding against the base of the post. The back side of the piece should sit flush against the post, with the angled cut moving out diagonally from both outside corners.
Step 6
Secure the piece with your nail gun and finish nails, shooting in two of them at a downward diagonal angle into the face of the molding.
Step 7
Measure and cut the molding for the other three sides of the post. Secure each one in the same manner, so the mitered ends butt to each other off the corners. Caulk around each edge of each piece of molding.
Step 8
Repeat the whole process, but upside down, to trim out the tops of the posts, where they meet the porch ceiling.