We see this question a lot: can I use the seat belt that comes down from the ceiling/roof of my SUV to install my carseat? Sure! Of course, double-check the seat belt section of your carseat owner’s manual and the child seat section of your vehicle owner’s manual, but as of this writing, I haven’t come across any combinations that disallow it.
First you have to assemble it to make it a usable seat belt. When you pull it down from the ceiling—or, in the case of some sedans, down from the top of the back seat near the parcel shelf—you have 2 latchplates (the metal male end) which will make your lap/shoulder belt. The end latchplate is usually smaller, looks different, and gets buckled first on the same side from which the seat belt comes from the ceiling. Now you have your lap/shoulder belt and can use it like any other seat belt.
To stow that seat belt out of the way, use its latchplate or a key to unbuckle it and retract it back into the ceiling. There will be a slot on the side of the smaller buckle which will pop the smaller latchplate out. After unbuckling it, let it retract and you’re ready to go!