Wipe the surface gently with a clean cloth and warm water containing a very small amount of soap. Use water sparingly. Dry with a soft, dry cloth. This should remove surface dirt.
Mix a solution of white spirit and linseed oil in a 10:1 ratio and apply liberally to the surface. Leave for two minutes, then wipe off with a clean cloth.
Gently rub the surface with very fine steel wool dipped in the white spirit and linseed oil solution, following the grain, to remove any of the remaining dirt. Wipe any residue off with a soft cloth before it dries.
Rub with a dry cloth and then wax with a good quality beeswax, making sure you thoroughly wipe off the wax afterward.