I want to make a wedged through tenon. The mortise is 1/4" wide by 3/4"
long, and the thickness of the piece is about 1/2".

I had no trouble chopping the basic through mortise with a 1/4" motrise
chisel. Just like a regular mortise, except I kept going :-) The
problem is how to cut the angled ends? I tried just paring the square
ends with my chisel, but the result was a mess. Any hints for how best
to do this?