First dry-fit your mating pieces together, as you have to draw a witness mark across both pieces. It helps later on if you draw the line as finely as possible.
The photo on your right shows me holding the tenon stock in the center of the wood. I judged this by ‘eye’ – and then you have to mark the witness line across both mating pieces, using the center of the tenon stock as your guide.
Carefully line up the witness mark to the edge of the ‘half moon’ on the jig.
Clamp the jig to the piece of wood using a bench vice or workmate. You could use a clamp, but this way both the jig and the work piece are secured.
Set the jig to the ‘A’ position by loosening the knobs and sliding the guide block to the left. (Three holes should be showing).
Make sure you tighten up the knobs before drilling.
Using a 3/8" or 9.5mm twist drill bit and masking tape to act as a depth stop, drill the three holes.
Undo the two knobs again and move the guide block to the ‘B’ position so that two holes are showing, then re-tighten the knobs and drill the two showing holes.
The instructions say to slow the drill feed rate when drilling setting "B" as this will produce a cleaner mortise.
The instructions also suggest to re-drill setting "A" after you’ve done setting "B" for a clean mortise and this helps to clean out the drill shavings from the mortise. This only takes a second and you are left with a much cleaner mortise.
Unclamp the jig and the workpiece.
Follow the same steps onto the other mating piece.
Cut the tenon stock 1/8" (3mm) shorter than the total depth of both mortices. The 1/8" will allow for the glue.