To restain your stair banister, first choose the color stain you want. Then, using many dropcloths and pieces of plastic sheeting, cover the staircase and any furniture that may be underneath. Next, using a weak solution of paint/varnish remover and a small brush, plus a clean cloth, cover an area of the banister and let the mixture soak in for a few minutes. When ready, you should be able to wipe the mixture away with a rag, or you may have to use a brush to clean any remover. After removing all of the paint/varnish, wipe down the banister with a damp cloth, and dry it with a lint-free cloth. Next, wait about 30 minutes to be sure everything is dry, and then apply the stain you have chosen. It may take several coats to achieve the color you want. Remember to make sure the room is properly ventilated before you start.