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To create the base around each column, measure three sides of the post. Transfer these measurements to a 5" base trim molding. Cut 45-degree angles on the molding for the corners.
Add wood glue to the corners of three sides of the molding, and join pieces together. Assembling three pieces will allow you to dry-fit the pieces and check the angles. Add the three pieces to the column.
Measure for the last piece. Cut, test fit, glue and nail it to the other three pieces of molding.
Use a torpedo level to make sure the entire trim box sits level on the post -- not the porch. Nail the entire unit to the column using the trim nailer.
Create the capital at the top from 1 x 4 stock. Clamp a board to the work surface, and use a router to cut a decorative edge into the wood to create an ogee profile.
Measure the sides of the columns, and cut on the miter saw. Nail them into place.
Add the trim pieces against the canopy underside. Add the trim around the top of both columns to add top capitals.
Fill all the holes with indoor/outdoor wood spackling. Caulk all the seams along the base and capitals -- and caulk the corners of the 1" trim.