Picture of the three bases after making the mortises / tenons and testing each fit prior to glue up and assembly.
Before laying out and creating the mortises the back legs need to be planed as a pair together to insure the same dimensions and surfaces are consistent. Below is a picture using a square to ensure the area to layout the mortises are 90 degrees to the bottom leg surface. As you plane the pair of legs to the final surface finish you want to check this periodically.
The mortises for the back legs are 3/8″ from the outside edge and exit the rear at 5/8″. Below is the process I use for creating the tenons for the side rails.
Notice the smooth surface created by striking the layout line with a knife. This makes for easy chisel work to finish the surface and a clean fit upon assembly. Almost always we undercut the surface as this ensures a tight fit and appearance. Below are pictures of cutting the tenons to size. We cut a bit oversize and sneak up on the final fit with a plane , rasps and floats.