I've done most of the rooms in my house.
1) With a punch, recess all the nails in the boards
2) Seal the room
3) Start sanding with the coarsest sanding belt
You may have to sand across the grain of the wood, as old floorboards don't stay flat, but become n or u shaped.
4) When you've done the whole room, put a smoother belt on and sand them again
Because until you varnish/stain them, you don't see the patterns left by bits of the belt not reaching the wood. Very important this.
5) If required, get on your hands and knees and sand the bits the machine can't quite get too
6) Sand the edges with a small belt sander, or a sanding disc on your drill (if the latter, make sure you clean up with smooth sandpaper to avoid circular marks)
7) Hoover the room up
8) Stain whatever colour
Its a big, messy, noisy job, but if your boards are in decent condition its well worth it. Be warned though, the room can get colder with a draft coming through the boards. You can stop this by insulating the undersides of the boards.