I started by drilling holes with forstner bits, trying to match the correct drill diameter to the right location on the designated mortise.
Next I used bench chisels of different widths to clean the mortise. I kept checking and matching the key to the progressing mortise to achieve a perfect fit.
I did not create a through mortise. The mortise goes down to about 3/4 of wood thickness.
Sometimes I had to use a swan neck chisel to extract and pare wood chips form the bottom of the mortise.
I have an old socket chisel (1") wide that I keep very sharp. This chisel can cut trough end-grain like butter!