Place plastic over the non-stained area of the wood, and tape into place. This will prevent you from accidentally bleaching any areas other than those which are stained.
Put on your gloves and protective eye wear.
Pour full-strength bleach onto the stain until the stain is well-saturated. Let sit for 15 minutes. Repeat if the stain is still visible.
Allow the bleach to dry on the wood once the stain is lifted.
Pour equal parts white vinegar and water into your bucket. Use the sponge the rinse the bleached areas of the wood. The vinegar will neutralize the bleach.
Let dry for 24 hours before sanding and refinishing the wood.