Hey Sara... if your henna is fine enough, an applicator bottle will do just fine. If there are some bigger pieces of leaf matter, then it will clog. Did you get Light Mountain Henna? or the stuff in the little plastic jar... forget the name of it. I haven't used one of those in years, since I switched to skin/temp tattoo quality henna, so I don't remember how consistent they were. I usually rub some oil or oily lotion around my hairline and ears, but immediately wash those areas when I'm done applying the henna anyway. Hennaforhair.com
is the place to go to learn about henna. The head of the site is actually working on a PHD in henna (!), so it's really informative.
eta: I'm not sure why I'm wearing sunglasses.