First, pound the rebar in at the right locations, as determined by the layout. I put them every 5 feet (3 per side). Keep them straight up and down, and keep them even, or the ribs will be skewed. Always cover them with cans or PVC when you're not actually pounding on them, or they can bite you or someone else.
Next, take 2 10' pieces of PVC and put them into a sleeve (making sure it's halfway over each piece using the markings).
Lift them up, and slide one over a rebar stake. It helps to have 2 people for this, but it can be done with one. The PVC is really floppy, so be careful not to bean anyone.
Finally, take the other end and push it over the opposite stake. The sleeve should be securely in place.
Repeat for each rib.
The top support keeps the end ribs from bending in towards the middle rib when the fabric is pulled tight.
Take another 10' piece of PVC and lay it along the top of the ribs. Use innertube straps to secure it along each rib. They don't need to be super tight, just stretched some. I've never had these slip.
If you're doing 20' or more, overlap the top supports by a few inches. You can either tie them separately or tie them with the same piece of innertube.