All you have to do is take the ramscoop motors mounted on the rim walls and point their exhausts tangentially- and wait. StrawberryFrog's write-up is good, but is perhaps a little short-sighted about solving some of the technical issues involved in building one of these suckers- for example, spinning up the ringworld is an easy problem to solve. It's probably a more practical concept than the Dyson Sphere, but vastly more efficient than these lame ball-shaped worlds we use now. In other words, it's a gigantic concept to get your head around. A meteor defense system would be necessary (in the book this was accomplished by using the star as an astronomically sized gas laser). Oceans would have to have a plumbing system to keep all the soil from winding up at the sea bottom. A series of Bussard ramjets would keep the Ringworld on-center. You could put a series of electromagnetic rings on the rim to create an em catapult for launching ships with. Any oceans and mountains would be curved directly into the material the ringworld was made out of- you'd get a bas relief by looking at the other side. A set of 1000-mile high walls on the edges hold the atmosphere in. You produce energy by lining the sunward side of the squares with solar collectors, and beam it back to recieving dishes on the ringworld as microwaves. For example, you'd put shadow squares in an orbit closer to the star, which would allow for day-night cycles. Larry Niven has fleshed out this concept to an amazing degree. Or, as he put it, by the time we need one we'll know how to build it. This would give us roughly three million times the usable surface area of Earth.

Niven estimates that one could be built of this size out of roughly the mass of Jupiter. This would cause approximately Earth-like conditions on the entire inner surface of the ring. It would have a radius of 1 AU, and be spinning at 770 mi/sec to give it 1 Earth gravity. The concept is of a gigantic ring encircling a star, with the living space on the inside. A sort of uber-world envisioned by Larry Niven.

Ringworld engineers