Begin by producing a through mortise and tenon in the normal way
Then prepare some wedges, approximately 4-7degrees, 3/4 the length of the tenon, and a slip fit across the mortise width.
Make a saw cut towards each end of the tenon, about 3/4 the way to the shoulder. These will accept the wedges
Widen the rear of the mortise on each end, by the thickness of a wedge (fat end) minus a saw kerf.
Pare the widened mortise ends down to the original width at a point 3/4 the way towards the front of the mortise. Creating a dovetail shape inside
This will allow the wedges to fit tightly in from the rear
Glue and insert the tenon as normal. Then apply glue to the top of the saw kerfs and the wedges...
...and drive the wedges home, being careful to strike them squarely, so as not to break them
Once cured, the excess tenon length and wedges are planed flush
Now watch the video: