Strip the finish from the wood, if there is one. Spread chemical stripper with a paintbrush, scrape it off with a putty knife and steel wool, then wash off the stripper with water and let the wood dry.
Sand the wood with 100-grit sandpaper to remove stripper residue. If the wood didn't have a finish, it's still a good idea to sand to open the grain and make bleaching more effective.
Use a two-part wood bleach. Part A consists of sodium hydroxide, which opens the wood grain, while part B consists of hydrogen peroxide, which reacts with the sodium hydroxide to bleach the wood.
Paint part A liberally on the wood, which will darken as you spread it. Immediately after you've covered all the wood, paint it with the part B solution. The secret is to get the part B on the wood while part A is wet. Let the bleach work overnight.
Mix a 15 percent solution of white vinegar and water in a bucket and wash the wood thoroughly to neutralize the bleach. Let it dry for three days, then sand it and spread a coat of sanding sealer or clear finish diluted with 50 percent thinner. Let the sealer dry, then sand it by hand with 220-grit sandpaper.
Mechanically distress the wood by denting it with a hammer, hitting it with a chain, or burning it with a blowtorch. Cut the head off of an 6d finish nail and use it as a drill bit to simulate worm holes. Make the holes in the soft part of the grain, the way worms would.
Add a small amount of white pigment to a clear finish to make a glaze and apply the mixture to the surface with a paintbrush or by spraying it. Let the glaze dry until it's tacky, then wipe it away from edges and corners with steel wool to make those areas darker.
Spread a final coat of clear finish to seal the glaze. Use a matte or semigloss finish to dull the wood and make it look older.