Prepare the ends to be joined square and true.
Use a marking knife and mitre square to knife the mitre lines on each piece.
Connect the short side of the mitre knife lines using a marking knife and try-square.
Select a mortise chisel about one third the thickness, and set the pins of a mortise gauge to the width of the chisel.
Mark the tenon cheeks on one piece, on the end and between the end and the knifed line on the edge.
Cut the full mitre through on the other piece..
..and mark the mortice cheeks with the gauge..
..across the mitred end.
Using the mortise chisel, chop out a mortise in the mitred end.
The try-square helps to keep the end cut vertical.
Once the mortise is completed, the mitred end can be shot precisely to the knifed lines.
Saw the tenon cheeks on the other piece..
..keeping the saw on the waste side of the gauged lines.
Line the components up, and..
..transfer the position of the mortise ends on to the tenon component.
Saw the ends, and the shoulders, of the tenon..
Now pare the shoulders, all round, to the knifed lines.
Fit the tenon to the mortise. If your cheek cuts were on the line, then you should only need to clean off the saw marks for a good fit.
Insert the tenon fully, and note the size of the gap..
..before sawing the tenon slightly short, to allow for excess glue.
Do a final test fit, before assembling with glue.
If all's well, the result will be a square corner, with tight mitre lines.
Now watch the video: