Step 2: Legs!
Attach the braces the same way; start flush at the end of the "X" tube, pin, then pivot down until you hit the leg. Clamp, drill, glue, and bolt. You may want to scribe the tab cuts on the braces to fit more tightly against the legs. All the compound curves and conic sections involved in cutting cylinders can be tricky, so just do your best with a pencil, a box cutter, and your eye.
Repeat until all four legs are attached and you have two identical frames with two legs, two braces, and one cross-piece each. Lay the saddle joints across one another and bolt through. Hand-tighten the "X" bolt, as it will need to be removed later in the process.