To start you gather a handful of hair from the top of the head and divide it into 3 sections (like for a french braid). Cross the right and then the left over the center like you were starting to braid. Then add a little hair to both the right and the left sides.
Twist both of the outside sections to the right. Then cross the right strand over both of the other sections. *Again add for hair to both the right and left sections and twist them to the right and then cross the right section over the others. Repeat from the * until you have roped all of the hair.
This is the finished product. If the hair you are doing is quite long then you can have a long rope past the french roped section. With this hairstyle it feels like you do need a third hand but it gets easier.
Anyway with 2 girls, a helpful book like this works for me!