Make deck post covers using 8-inch long sections of 2-by-8 lumber and a table saw. Set the table saw blade to a 15-degree angle to create the cap's peak, or experiment with different angles using scrap lumber. Place the 8-by-2 lumber on the table saw, with the 2-inch side resting on the saw. Clamp a piece of scrap lumber to the 8-by-2 for extra stability. Feed the lumber into the saw.
Repeat this procedure for each deck post cover, cutting all four of the lumber's sides to create a beveled edge with a peak. Sand the finished covers, and use a rag to remove any dust. Paint or stain the caps the same color as the rest of the fence.
Use wood glue to fix the covers to the fence. Apply the glue to the covers' backs and the posts' tops, and press the cover down on top of the post.
To make post covers using a handheld circular saw, make a jig to hold the lumber in place, and provide a guide for the saw. Set the saw blade to 15 degrees, and clamp the lumber in the jig so that it stands on its short edge. Cut each side, rotating the piece of lumber as you go.