Most stores stock wood stain in every shade of brown despite a manufacturer's spectrum that includes vivid reds, deep blues, and even bright whites. Picking colors is definitely the fun part. But refinishing a floor also involves cleaning and maybe some repairing aside from sanding.
Remove deep blemishes that don't respond to light sanding by applying full-strength bleach, washing between repeated coats until the wood has a neutral tone. After the entire floor is sanded, prime these dull spots with a thin coat of stain to bring them close to the surrounding floor color, then use fine sandpaper to reduce any raised grain.
Burn marks. Remove surface damage by scrubbing with a household cleanser, or simply shave off the burn with a sharp scraper or chisel. Don't oversand, creating a trench that the floor sander won't reach. Treat deeper burns as a deep set stain with bleach and stain primer as above.
Pet stains. Sanding and bleaching should remove the discoloration. To deal with odors, try one of the proprietary mixes sold in pet stores, or soak the stain with hydrogen peroxide and cover with a cloth soaked in ammonia until the stain fades or the solution dries. (Never combine bleach and ammonia, which produces dangerous fumes.)
Cracks. If you want to fill some major cracks, first test one or more fillers—where the couch will be, for instance—to see how they accept stain compared to the surrounding wood.
Nail heads. Though most flooring is installed with concealed nails, check for slightly raised nail heads before sanding, particularly around the perimeter and any patched areas of older floors. Slide a metal trowel or rake over the wood, listening for a ding of metal on metal.
Lightening. If you plan to lighten a dark floor, the new stain color will be truer and brighter (not a muddy mix) if you first strip any sealer and wash the floor with a solution of bleach and water. (More bleach means more lightening.) That will create some slightly furry wood grain. But when you're planning to sand the entire floor in any case, who cares.
Sanding. To handle this job yourself, rent a large drum sander and a rotary edger, plus a supply of belts. Some damaged floors require a set-up pass with medium-grit paper, but for refinishing many need only fine-grit. Look for a machine with a tip-up clutch. It has a lever on the handle to make the drum rise so you don't have to tip up the entire machine. Also make sure you have electrical circuits to power the machine. Some heavy-duty sanders draw 25 amps and require a 30 amp circuit.
Here's the basic sanding sequence:
After sanding the main area, working with the grain, use the edger to sand the perimeter. Try to keep the edger moving, blending the rotary sanding tracks with the straight-line tracks from the drum sander. After finishing, you may see where the different patterns meet. Although it's an extra step you won't find in most how-to books, if you want a really nice-looking job, take the time to blend the edger tracks with a hand-held, random-orbit sander. You might forgo the effort in rooms where the perimeter will be hidden by furniture, but not in halls and spaces where the six inches or so of edge-sanded floor would stand out.
After vacuuming or using a tack rag to clear traces of dust, apply stain according to the manufacturer's directions. But it's wise to test the intensity in an out-of-the-way area. For full depth of color, you may have to let the stain soak in a while before wiping, but keep a wet edge to avoid lap marks. Quicker applications generally produce lighter hues. To protect the finish, you could apply wax, but better yet is two coats of polyurethane.