Cut three or four lemons in half with a sharp knife. Wash them first if you want.
Squeeze the lemons into a clean cup or bowl. For really short hair you only need a lemon or two. There's no exact formula for this, but the more hair you have, the more lemons you're going to need.
Pour the juice through a mesh strainer into a clean spray bottle to make sure you won't be picking pulp out of your hair and seeds out of your ears.
Fill the bottle with warm water and swirl it to mix the lemon juice evenly. This will ensure that you don't get patches of lighter hair where the juice was concentrated.
Spray the lemon water onto your hair, coating it well, and make sure you get the layers underneath the very top one.
Turn the blow dryer on and aim it at the top of your head. Work your fingers though your hair to separate the strands as you move the blow dryer from the top of your head down toward the ends of your hair. Starting at the top of your head will give you a more natural, sun-lightened look.
Wait a day between treatments; the combination of lemon juice and heat can be drying.
Never add bleach or hydrogen peroxide to your lemon water; both chemicals can damage your eyes as well as your hair.