The legs are attached by screwing from the "inside" of the 1x4s, before attaching "sandwiching" the 1x4s around the 2x4 center of the horse.

The top section has two 2x2 legs with 1/2" holes drilled in them at 2" intervals. A 4" section of oak dowel fits into the holes in the legs to set the height of the top of the sawhorse.