line up the flat face of a base panel to the back edge of the bottom panel and then the second base piece needs to be about three inches from the other end of the panel. you can then glue these pieces in place and drill pilot holes to secure the pieces together with screws.
attach the side panels you have to the base you just assemble with a glue and screw process. make sure the base and all edges are flush and be sure you get the right and correct 90 degree.
once again, you need to fit and glue the upper brace so that the flat pane is flush the 