The “mortise” is the recess where the tenon goes. It can be cut by hand with a mallet and mortising chisel, or mechanically with a mortising machine and a hollow chisel bit.

You can use a mortise marking gauge for this, or just a sharp pencil and a square [6]. Also, mark the lines below the mortises that determine the start of the leg's taper.

A mortise cut in a piece of 7/8-in. wood could be either 1/4-in. or 5/16-in. thick, as long as you have both size chisels to choose from. The depth of a mortise should be at least twice, and ...