Lay newspapers all around the piece that you're going to stain. If there is any hardware on it such as knobs or hinges, remove them with a screwdriver.
Sand down the entire surface of the wood with your vibrating sander and heavy sandpaper, taking off the old finish and the top layer of the wood. Vacuum up the dust.
Sand the cabinet a second time, with medium sandpaper, then a third time, with fine sandpaper, getting it smooth and clean. Vacuum up all the dust.
Use your paintbrush to apply the stain to one section of the wood, covering a few square feet. Brush the stain on thickly enough that it pools.
Let the stain sit for one minute, while it soaks into the wood. Wipe up the excess stain with a rag.
Assess the surface. If it's not dark enough, repeat the process and let the stain for the next sections sit for two minutes instead of one. Do the whole piece, adjusting the length of time for the stain to sit as needed.
Let the stain set for 24 hours. The wood is now ready to be sealed with gloss.