It just snaps on in most cases, and I believe your talking about a pontoon boat so it will have several support poles that normally fit into vents, and the poles will be different lengths. I took some 1/2 wide PVC taps and color coded mine so I don't have the adjust the length ever time. Mine has 6.
If you bought it used and found the supports are missing it is critical that you use something to support it. The guy down the block from me didn't follow this advice and there is a small glacier forming on his sagging cover.
