Stair treads probably get more---and more intense---foot traffic than just about any other surface in your home. Stained-wood stair treads can become worn and discolored over the years as a result. When you re-stain, you also will have to re-gloss over the stain, which looks good and protects the wood. It's not a difficult job, but it is time-consuming because of the drying time necessary between coats of stain and gloss.
Starting at the top tread, sand the tread surface with your power sander, which should be loaded with the heavy-grade sandpaper. Go around the front and sides of the tread as necessary. For tight spots around the balusters, fold up a piece of sandpaper and do it by hand. Sand until all the gloss and most of the stain is off, and you're just getting down to bare wood. Move to the next tread and continue until all are done. You will have to frequently switch out the sandpaper as it becomes clogged with gloss residue and sawdust.
Repeat Step 1, but this time load your sander with medium-grade sandpaper. Use it to remove any remaining stain on the wood and to smooth out the surface. Thoroughly sweep and wipe down the surface. With your painter's tape and newspaper, tape off the areas around the treads (surrounding walls, risers, balusters).
Put on your rubber gloves. Working from the top step, generously apply the stain with a brush. Put enough of it on so that it pools a little in the middle. Let it sit for about two minutes, then wipe it down with a paper towel. If it's not dark enough, reapply the stain and let it set another couple of minutes, taking note of the total time the stain is sitting on the wood. Let it sit for that amount of time on each tread as you proceed down the staircase.
Allow the stain to dry overnight, then dust it well. Open your gloss and gently stir it. (Don't shake the can.) Working from the top step and wearing rubber gloves, slowly apply the gloss to the tread, using long, slow strokes with the direction of the wood grain. Look out for the formation of bubbles and drips, gently brushing them out as they happen. Repeat for each step, then allow to dry overnight.
Starting at the top step, gently buff the dry gloss with fine-grade sandpaper by hand---not with the power sander. Sand just enough that the shiny surface becomes a little cloudy. This will allow the next coat of gloss to stick. Thoroughly wipe down the steps, then apply the next coat of gloss as in Step 4. Repeat the sanding and glossing process the next day for the third coat.