A floating hot tub cover is a quick and inexpensive way to protect your hot tub from random objects, and also to provide a layer of insulation to help retain some of heat. Since it floats, installation is a breeze, and if the cover isn't cut to fit your hot tub already, the only tool you'll need is a pair of scissors or a cutting knife.
Sizing the Cover
If you didn't purchase a cover pre-made to fit your hot tub size, you'll need to cut it yourself. You'll need to be careful not to cut too much, however. Lay the cover in the hot tub as centered as you can, and cut away layers of the outer edge until it fits snugly in the tub. Again, be careful to take small layers out until it fits; you can always cut more, but there's no way to reattach a piece that was too large.
Once it is Installed
Once you've cut the cover to size and placed it in the tub, the only thing you really need to do is make sure to get rid of any water pooled on the surface. The water's supposed to be underneath the cover, not on top of it.