Many people have developed simple instructional home treatments to apply onion juice to their hair.
For the most basic application:
Peel onions. Chop and dice into the finest, smallest pieces possible.
Extract juice. You can use a food processor, blender, juicer, grater, or press to make this easier.
Massage onto scalp and into hair. Leave it in to work for at least 15 minutes, then wash hair with shampoo afterward as usual.
Some people may avoid using onion juice for their hair due to its strong smell. For this reason, some have suggested simple recipes to help curb the onion smell.
Here are some ideas:
Mix onion juice with about 1 tablespoon of honey.
Mix juice with a few drops of essential oil of your choice. (Good options include lavender, eucalyptus, orange, lemon, or ginger.)
Mix juice with about 1 tablespoon of alcohol to mask the smell.