Wet hair to prepare it for deep conditioning. Use warm water and wrap your hair in a towel until ready to apply the mayonnaise hair treatment.Scoop out two or three tablespoons of mayonnaise into a bowl. How much you'll need will depend on the length of your hair. For short hair, start with 1 to 1-1/2 tablespoons. For medium length/shoulder-length hair, start with 2 tablespoons, and for long hair, scoop our 3 tablespoons.Apply the mayonnaise liberally to the scalp rubbing it in with the fingertips. Apply more mayonnaise along the length of the hair until all of the hair is coated.Wrap a plastic bag around the hair, being careful to keep the bag away from the face. The bag will trap the heat from your head allowing the mayonnaise hair treatment to work deep down.Leave the mayonnaise on for 20 minutes then rinse thoroughly.