Hi there glitter
I'm also self employed (for about 5 years now) and this will be my first baby. I also had a bit of trouble getting my head around maternity allowance as, whilst my work has generally been fairly regular, I have chosen to work a lot less in the last 12 months due to undergoing IVF and giving me and my baby the best possible chance when I have managed to sustain this pregnancy (22 weeks and counting, eek!).
Have you had a look at this DWP document on the government website?
www.gov.uk/government/uploads/system/uploads/attachment_data/file/520765/ma1-print_.pdf
I found it helpful to understand how the Test Period is calculated etc. Page 10 in particular, plus the schedules after page 18, are particularly relevant to those of us seeking to claim Maternity Allowance as we're self employed (as opposed to claiming MA because you're not entitled to Statutory Maternity Pay from any employer, despite being an employee).
I'm too early to submit my claim yet, but my understanding is that even though my earnings are a bit sporadic this year, I will be treated as having earnings as £30 a week in the relevant Test Period (see the schedule of the doc for your Test Period) because I have made at least 13 Class 2 NI contributions in the Test Period. This is how I understand the guidance on page 10 of the doc.
I think if you're properly registered as self employed with HMRC, it's actually a lot easier than first appears when you read all the stuff. They'll be able to see when you registered as self employed (to see if you qualify for the lower rate of MA) and if you've paid the requisite amount of Class 2 NICs to qualify for the higher standard rate of MA.
At least I think that's how it works! I rang up the HMRC on 0300 200 3300 to chat it through as I also found the info confusing, and found them to be very helpful. I think even if you've not paid enough Class 2 NICs in the Test period to get the standard rate, HMRC give you the opportunity to pay those contributions with an in-year NICs bill so you can claim the full whack. This is what I was told on the phone and what the MA1 application form seems to say in the self-employed section.
In terms of documentation, my understanding is that for the self-employed, you only need to send the form MAT B1 that your midwife/hospital will provide (I got mine when I had a hospital appointment with the consultant after my 20 week scan, which was actually at 21 weeks). I don't anticipate needing to send bank statements etc to prove payment as it looks like I'll be deemed to have the requisite earnings under page 10 of the guidance.
I hope this helps! We shall see when I come to put in my own application next month :-) If in doubt though, just call HMRC to chat it through. It turned out much simpler than I'd imagined!