How do you restore a 1885 shotgun that has been in the attic since 1910?

First, ensure that it is not loaded.

The wooden stock and forearm can be cleaned with a soft cloth, and any good quality furniture cleaner/ wax.

The metal parts are best "restored" with a clean soft cloth, light oil, and a LOT of rubbing. Do NOT use steel wool or similar abrasive materials. If it is rusty (not just an even brown color, but rusty) and the cloth and oil are not enough, find a COPPER Chore-boy pot scrubber at the grocer. Wet it with light oil, rub gently, followed by rubdown with soft cloth and oil. Be sure to wipe excess oil off before storing it.