Stripping paint or varnish off any piece of wood is not difficult. However, with a musical instrument, while it's easy to do, some things need to be kept in mind. Guitars of any value are rarely refinished; no matter how battered, they are still worth more as they were when first sold. Also, it's difficult to come close to the original quality of finish that is applied at a proper factory with skilled luthiers. Refinishing an acoustic guitar can severely affect or even ruin the tone, so we'll assume for this article that we're only discussing electric solid body guitars.
Remove all hardware from the guitar: the tuners, the pick guard, the pickups, the controls, the strap pins. Finish stripper is made of strong chemicals, and could severely stain or discolour metal parts, or ruin electronics. Remove the neck if it is a bolt on. Remove the bridge.
Set up shop in a well-ventilated area. Lay down lots of newspaper. Put some of the finish stripper in a bowl. Lay the guitar down carefully beside the bowl. Apply masking tape to any trim or other non-wood areas such as binding.
Paint on the finish stripper with the brush. Put on just enough; it's best not to have any run down into cavities where electronics or tuners will go.
Leave the finish stripper for the prescribed amount of time. Once this has passed and the finish has gone soft, start to remove it with the scraper. Be very careful if the guitar top is contoured; it is easy to scar the wood with the scraper. Get as much of the finish off as you can.
Use the sandpaper to remove the last bits of finish and to smooth the wood, as the finish stripper will raise the grain and leave the wood looking slightly fuzzy.
It's possible to remove the finish from a guitar purely through sanding. However, guitar finishes tend to be hard and thick, so this could take quite a lot of time and quite a bit of sandpaper.