updated by @jambi: 01/13/15 09:05:18PM
I keep a plastic cup in the shower. When I use my Bronners, I squirt a tiny bit (think less than a tablespoon) and then fill the cup with water. Then I just pour all over my scalp! It's the scalp you need to focus on more than the length of your hair; the bubbles will work their way down.
I have found that I get itchy faster when I use Bronners -unless- I follow with an apple cider vinegar rinse (and a good cold water rinse never hurt either).