I typically use loose tenons for mortise and tenon joinery. I find them to be much faster than a traditional M & T, plus it’s much easier to get accurate shoulders. I use my Inca slot mortiser to cut the mortises. It’s fast and simple to set up and it cuts a mortise in about a minute.
To lay out the mortise on the leg, I start by drawing the profile of the apron. In this case, the outer face of the apron is angled. I could get the angle from my drawing, but I prefer to get it by eyeballing what looks good as I draw it on the leg. This angle will become the actual angle on the apron.
Once the apron profile is drawn, I draw the mortise. I typically leave 1/2″ of material at the top of the leg, but in this case I cut it to 3/8″. Because of the stresses on the legs and aprons as the table is expanded, I wanted all the glue surface I could get. Smaller than 3/8″ could result in failure of the short grain at the top of the leg, so I rarely go less than that.
The photo below shows the layout. Note that I only draw the complete layout in one location.
The other locations are only marked for the length of the mortise. Why? That’s all I need. When I use my slot mortiser to cut the mortises, the height of the mortise above the table is fixed by the machine. Once set, it’s the same for all of the cuts, so why waste my time marking more than I need to?
To maintain accuracy, I mark the mortise lengths using a combination square. I use it like a depth gauge, with the head resting against the top of the leg. The END of the blade is set to mark the lengths. Once set for a given distance, I mark all the legs AND all the apron ends. That way, every mortise is in exactly the same spot relative to the top of the legs. Here’s an example of the apron end layout (this isn’t an actual apron).