This part is pretty simple.
Have everything pre-assembled without dowels and wheels flat on a table outside or inside a well ventilated area.
IMPORTANT. PVC cuts (2.) and (4.) should not be touched by glue until everything else is glued together and cured or else you wont be able to true the wheels alignment!!!
Before the frame cures, save time for straightening by gluing only one end of cut pipes to a fitting throughout the whole assembly.
Best way to start out is
Central mount fittings clamp to table and use scrap pipe to make sure its at 90 degrees.
One flusher stub for each right-angle.
3+6 with T fitting.