Here are the steps I use to fit an armrest to the armchairs. As usual start with a mortise.
Use a sanding stick that is nearly as wide as the mortise. Attach 100 grit sandpaper to only one side of the sanding stick.
Use the sanding stick at a slight angle to clean up the mortise. The angle in the picture is exaggerated for clarity. A slight angle will prevent damage to the exposed side of the mortise, as well as make assembly easier.
Fit the tenon as you go until you have a nice snug joint.
Mark the through tenon.
Form the pyramid on an oscillating belt sander. Tilt table to 15 degrees. Set miter gauge to 90 degrees.
Final tenon shape.
Finally, hold the armrest next to the tenon as you mark the notch for the back leg. Cut the notch with a bandsaw, and install the armrest with a screw and decorative peg.
-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush