Collect 2 pieces of wood, soft would be the best, both of equal sizes. Divide each piece of wood into 3 accurate segments using an accurate ruler, use a pencil to draw the face side and face edge. Next use a tri-square to mark the half way mark on the side edge of both pieces of the soft wood. Use a Stanley knife to make a deep cut on the two pencil marks on the face sides of the two pieces of wood making sure to use a ruler to guide the knife - we do this so that the saw cuts later are very accurate. You’ll know if this is right if you place one of the pieces on top of the other making sure you can only just see the lines from the width of the wood. Now use a gauge to accurately mark the pencil lines made on the face edge making they are visible from the eye. Then using the grooves made by the Stanley knife earlier cut down the pencil line of face edge with a Tenon saw, saw down till you get to the gauge line you made earlier on face edge. Then collect a bevelled edged chisel and manually work through the wood peeling of and mm a time remember to only go half way so you can spin the wood round and get an accurate cut leave about a mm before the gauge line so we can accurately use a Granny’s Tooth or router as its commonly known to gently peel off the final mm. Now the two pieces should slot into each other and you’re done, it should look something like this.
Bevelled edge chisel