Start by clearing the room and finding all the protruding nails in the floorboards.
Bang in these with a nail punch (and hammer) so you don’t damage the boards. If you leave any nails sticking up, they will break the sanding sheets, which is annoying, time-consuming and expensive because you usually pay for each sheet you use.
As well as hiring an industrial floor sander to sand the main part of the floor, you’ll need an edger to sand right up to the skirting boards - the two sanders can usually be hired as a package. Before using the machines, put on a dust mask and goggles, open the window and close the door, as there will be a lot of dust.
To start sanding, lift the drum of the industrial floor sander off the floor and turn on the machine. This avoids damaging the boards and letting the sander get away with you - it’s powerful, so be careful. Walk the sander steadily across the floor in a diagonal direction, but don’t linger in one spot because you’ll create ridges in the boards. When you turn off the sander, make sure the drum has stopped before putting it down. The edger is more straightforward to use, but back-breaking to operate after a while.
The coarseness of the sanding sheets you use will be determined by the state of the floorboards. If they’re black, use coarse sheets first, but be prepared to get through quite a few because they’ll clog up quickly. If the boards are in better condition, try starting with a medium-grade sheet, and end with fine for a smooth finish.
You won’t be able to get into the corners of the room with the edger because its sanding sheets are round, so use sandpaper or an electric detail sander. Getting the corners looking as good as the rest of the floor isn’t always easy, but is worth the effort.