Techniques
So, how do you henna your hair?
Next Step:

 I spooned on more henna and smoothed it down the length of her hair. 

Disposable plastic gloves are crucial because henna stains your hands! 

Have lots of towels around to contain the sloppage.  Henna washes out of dark cotton towels.  If you have white towels, bleach them to get the henna out.