No, there's no documenation on how to create a custom web form, other than what you see on the screen itself.
Our ambition was to make it wizard-based so that someone using it could follow the on-screen steps and build a web form without having to read a separate document.
I think for the most part we've done this, with the exception perhaps of the formulas in the 2nd last step, as they are pretty complex.
Everywhere else, though, we'd like to refine the steps so that they make more sense as you go through them. So if you're game for giving us feedback, maybe we can work towards that goal together.
Where exactly are you getting stuck?
Thanks for your feedback,