Hi! I am a doula and have been practicing for almost 2 years. Pretty much anyone can be a doula, really, as it's not a regulated profession...but if you want credibility you are best to take a training workshop through an established organization (ALACE, DONA, ICEA, etc) and then work for your certification. My certification work includes attending a certain number of births as primary support (not shadowing, but shadowing is also invaluable!), an extensive written exam (some short answers and about 18 mini-essays), written evaluations from mothers I've supported and/or nurses/OBs/Midwives who were at the birth also and a slew of self-evaluation write-ups. There is also a HUGE reading list of birth texts and books that are required - this can be expensive, as many are pretty much essential to own (and makes a great lending library to offer clients, too). After that I am required to take a certain number of continuing ed workshops or courses each year to keep my knowledge and skills current.
As for what the job entails, I don't have any liability insurance myself since the doula scope of practice does not involve ANY medical care whatsoever. My role is to be a resource for informed choice and education, emotional support, and physical support. You have to be willing to be on-call for several weeks surrounding each client's EDD, and make sure you are always available. You have to have on-call childcare, as well as backup childcare just in case. Backup doula support in case your children are sick or whatever and you can't make it. Other than that, you need the right attitude, demeanour, professionalism, and dedication!
I love, love, love my work as a doula and it's the kind of thing I would do even if there were no money in it. I am passionate about birth advocacy! If you want to know more, please PM me!