Once your main board is added, cut your trim board the width of the front of your main board and attach the board to the front of your shelf to finish off the look. I attached most of my trim boards with the brad nails and nail gun, but there were a few spots where the trim was bowed a bit, so I pre-drilled and used screws there to really keep the boards together (brad nails and glue should be fine for non-warped boards but screws are a stronger hold if they are trying to pull apart based on warping).