To measure the middle rail, align it so it is flush with one side and mark the length on the other side with a pencil. Add 1” (25mm) to this overall measurement as the rails will be through tenons, Paul uses the width of a square for this as most square beams or steel rules are 1” wide. Square this around each side using a knife.
The top rail is slightly different as it is a stub tenon, position the end of the rail ¼“ (6mm) away from the edge and measure ¼“ (6mm) into the the other side and make a mark. The aim is to have a top rail that is ½” (13mm) longer than the opening between the sides. Square this around using a knife.
Create a step down using a chisel on both rails and cross cut these to length.