Remove any patio furniture from your deck before you start.
Bang in any nails that stick up using a hammer, since they will damage the sander.
Cover any plants (such as hedges) near your wood deck to prevent them from getting covered with wood dust, using a tarp. Get eye goggles and a dust mask for yourself so you don't breathe in wood dust or get it in your eyes. This Old House suggests wearing ear protection, since the sander will be loud.
Place four 36-grit abrasive disks onto your random-orbital sander, one for each sanding disk. The abrasive sandpaper sticks right to the disk.
Hold the sander and turn it on using the power button.
Push the sander back and forth across the wood floor. Do not push down, just move the machine across the wood. Work in a curve; you don't need to follow the grain of the wood. Sand over the whole floor in this manner.
Turn the sander machine 45 degrees so it becomes a diamond rather than a square. Work over the entire floor again. This helps you get areas that weren't reached before due to the setup of the disks in your sander.
Vacuum up the wood dust you've created. By now you'll have sanded most of the desk down to bare wood. There will still be finished spots near the railing and in corners.
Attach 36-grit sandpaper to a detail sander. This Old House recommends using a model that plugs into your vacuum.
Turn on the detail sander and sand these corners that still bear finish.
Remove the 36-grit sandpaper from the random-orbital sander and change it to 60-grit sandpaper. Then sand over the entire floor as you did before with this finer sandpaper. This will remove scratches you made when sanding initially.
Replace the 60-grit sandpaper with 80-grit sandpaper and sand one more time. You deck boards will be entirely paint or finish-free and smooth, without scratches from the sander.
Vacuum again to remove wood dust. Now you can apply fresh stain to your bare wood deck.