Step 2: Legs!
Cut the same 3" slot in one end of four full-length tubes. These will be your legs. Bend the "tabs" created by your cuts to fit around the "X" tube, tight against the center saddle notch. Clamp in place to drill a hole. Unclamp, apply glue to the inside of the tabs, replace, and then bolt through. Don't over-tighten the bolts, as you can crush the tube. Use a clamp to flatten the tabs against the surface of the tube they are joining to until the glue dries. I rounded off the tabs with a box cutter and hit them with a little sandpaper to clean up the cuts.
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.