Use your biscuit joiner for mortising
I've never had much success hand-chopping mortises, so I tried removing the bulk of the waste with a biscuit joiner. Then, I simply chop the slots square using a chisel. The slots' walls help guide the chisel cuts. Result: perfect mortises.
You will have to do some math to calculate the thickness of spacers placed under the biscuit joiner to achieve the correct mortise width and positioning. Most biscuit-joiner blades cut approximately a 1⁄8 " kerf , so two plunges will yield a 1⁄4 "-wide mortise.
—Mark Buckley, Norwich, N.Y.