First, let's change some vocabulary. You will be a doula and not a "head doula" which implies that there's some kind of hierarchy and there isn't. You will simply be these women's doula and if you follow your heart and their cues, you will be fine. As a doula, the best tools you have are your heart and your hands ... ok, let's also add a bit of common sense.
I did not follow a doula around but then I began doing this before we were called doulas. It was the 60s, I was in high school and it was, "Why don't you call her? She helps women in labor." If your intentions are right, if your heart is right, how can you go wrong? You give comfort, you wipe brows and sometimes you wipe lady parts or bum parts or hold a puke bucket. You give water, you rub backs, you run tubs, you dry ladies off after the tub, you go get towels and more towels and more towels and more ... well, you get the picture. You just do. Stand back and see what sweet moments she can have with her partner. Take pictures of those moments. If the partner is hanging back, do more. If the partner seems to be able to handle al the physical stuff, you hang in the background and hold the space for them--adjust the lighting, run the bath, get those towels, take pictures, try to be one step ahead of what the both of them need.
I do have a contract and I don't insist on being paid upfront, but I do ask for a deposit which covers my prental time with them. I ask for the balance by the 38th wk but some people need a payment plan and make payments until well after the birth.
Take your cue for your energy from the motherbaby and you should be totally fine.