With each of the four side pieces ripped into the appropriate L-shape, the next step of these free barn door plans is to miter the corners and attach the trim to the door.
Set up your compound miter saw to a 45-degree miter, and cut one end of one of the four trim pieces (with the long point being along the 2x side of the board, and the short point along the 1x side). Then, cut a corresponding opposite miter on the adjoining next trim piece, and dry fit it onto the door. The two boards should align perfectly along the edges of the door frame, and the miter should match perfectly in the corner. If it doesn't adjust your angle on the miter saw and clean up the cuts. Continue around all four edges of the trim, dry-fitting all four boards.