Measure half way and cut the piece in half
Cut the Tenon out of one half of your wood
Chisel out the mortise
Make it fit
You may find that when you go to join the mortise and tenon that it doesn’t fit. If this is the case use your chisel and to either make the mortise larger or the tenon smaller.
If your hole is larger than the mortise then turn the piece with the tenon around and cut another tenon to the size of your mortise. Take your time and remember to cut on the WASTE SIDE of your line
You should be able to hold your mortise and tenon join upside down without it falling out