The ends of this hoop are simply pushed into the ground as far as they can go. The spacing of the hoops depends on a few things. First, if you live in a windy spot you'll want the hoops spaced more tightly. Also if the rain really comes down hard you'll want that extra support as well. If the cloche is being used to mainly provide extra heat during the calmer late spring and summer months, the ribs can be spaced at a fairly wide interval.
Cloches are usually informal structures.
I simply lay the plastic over the hoops, and weigh it down with water-filled jugs. One end is kept open for ventilation almost all the time. On sunny days you can open up both ends, or remove the plastic altogether. They run a bit less than a dollar each. Two companies that carry them are Territorial Seed Company and Raintree Nursery.