Choose the face you want for your cross rails and identify these by drawing a triangle across both the faces. With the top rail, measure ¼” (6mm) from one end for the tenon and make a knife nick, square this across the front face and down each side using the knife. This is called a bare face tenon which only has a shoulder on one face on the front edge.
Line up the ¼” (6mm) mark you just made on the end of your top rail with the inside of the side piece, use your knife in the nick to make sure it is right up against the side piece. Hold this in place and make a mark on the other end of the rail in the same position using the knife.
Set a single pin gauge to the thickness of your ½” (13mm) chisel. Put the stock of your gauge on the back edge of the top rail. Run the gauge line on the end grain and side to mark the bare faced tenon.
As the middle rail tenon is a through tenon, you need to mark ½” (13mm) from one end. If you have a gap between the end of the shelf and the sides, use a sash clamp to tighten this before you make the mark on the other end of the tenon.
Line up the pencil mark you just made with the outside edge and use the knife to make a mark on the other side of the side piece. With the rail still in the same position make a knife nick on the inside of the other side.
Remove the clamp and use your knife nicks to make a line all the way round, the distance between these shoulder lines has to correspond with the front edges of the sides or else it won’t match up, so take your time with this step. Use the gauge to mark out the tenon in the same way as you did for the previous tenons.