Lay out the center-line of the dado for the shelf, by ganging both legs together with a clamp.
Note: I use only a centerline as this eliminates the mistake of being on the wrong side of the line.
The dado jig I am using, wonderfully explained by the Wood Whisperer:
is clamped in place with a plunge router/ bushing setup.
Putting the boards back together, to double check that the dado lines up, here is the result:
With the dado cut, we can layout the mortises for the through tenons.
Score the outline of the mortise with a marking knife, to fight chipout.
Next up, how do you cut those mortises?