Cut the top panel board to size and attach it to the top of the cabinet with 1 1/4″ brad nails and wood glue. There should be a 1/4″ overhang on the sides and front of the panel and the panel should sit 1/4″ from the back.
Cut the side trim pieces to size and attach them to the sides of the top panel with 1 1/4″ brad nails and wood glue.
Cut the front and back trim pieces to size and attach them to the top panel with 1 1/4″ brad nails and wood glue.