If you've used too much shampoo and have some leftover shampoo in your hair you can usually get the excess shampoo out of your hair by massaging some apple cider vinegar into your hair.
Then just rinse out your hair once again or maybe twice and the leftover shampoo should be out of your hair.
Adding some baking soda to your hair can also wash the shampoo out of your hair or simply just rinse your hair 3 or 5 times until the leftover shampoo gets out of your hair.
Washing your hair using a clarifying shampoo can also remove extra shampoo residue and any other residue left behind in your hair.
To use the baking soda to remove excess shampoo from your hair just mix a tablespoon of baking soda into a regular amount of shampoo and wash your hair with it again.
Then rinse your hair out a few times and the excess shampoo should be removed from your hair and if not you may need to rinse your hair over again.