The first step is to remove the old seatbelt fasteners and the seat belts themselves, the latter being an easy task if you don’t mind having a bit of work done.
The rear seatbelts are replaced with the new ones, and the front ones are replaced too.
The seats themselves are a bit harder to get off, as you’ll need to get into the trunk, lift the rear seats, and then unload them.
This requires a bit more than simply pushing them off, however.
You’ll need the key, a screwdriver, and a bit patience.