Take a side piece and flush it with the end of the top piece, use the square and slide this until it butts up to the side piece. Remove the side piece whilst keeping your square in place and use a sharp pencil to make a mark.
The housing dado is positioned one board’s width from the end of the top and bottom pieces. Using the mark you have just made,, move your board over so that the opposite side is now against the first mark. One the side piece is dead on the first mark, make a second mark using the sharp pencil. This ensures the housing dado is an equal distance away from the edge of your project.Square your housing dado marks down the edge of your piece. Mark the inside of the housing dado with crosses to identify your waste wood, repeat on your bottom piece.
Square the lines across the edge and face on the second piece of wood.
Use the side piece to mark the length of the stopped housing dado.
The position of the side piece.
Use the gauge to mark the dado depth (still ¼” or 6mm). If you don’t have a gauge you can use the router plane blade as shown.