In my design the legs are attached to the arms with wedged through-tenons which will be trimmed flush with the arm’s surface. First I cut the tenons on the top of the legs and then proceded to cut the mortises in the arms. The pic below is of the first mortise, started with a forstner bit and trimmed square with a sharp chisel. The beer is for safety.
This pic is of the through tenon (before being drilled and slotted for wedges) through the squared mortise. At this point the mortise is just a square hole, cut 90° to the to the top of the arm.
To allow a wedged tenon to spread open (and give the joint it’s strength) the mortise now needs to be bevelled. I used a block of hickory cut at 3° as a guide and pared away the top of the mortise on the two sides I will be spreading the tenon towards.