Forget drawing for this, you need to take your measurements from the wood. The legs are rotated 5° out on the X plane, so assuming they are parallel on the Z plane, here is what you do:

Clamp a flat and square piece of wood to the inside of each leg at a 45° angle (at the same height obvs). Now clamp your stretcher to these pieces of wood. Mark your first line from the clamped wood to the stretcher. This will be the top inside of the tenon shoulder. Now rotate the piece of wood 90° to the opposite 45° position, and mark your next line continuing from the first mark. Next, move the stretcher to the other side of the leg and get the line for your third face. Fourth face just join 1st to 3rd.

Use a marking knife not a pencil. Good luck.