I might get to paint my first belly

Ok, so i'm so excited and terrified at the same time A local photographer just contacted me to see about painting a mum-to-be for a photoshoot. I'd love the chance and opportunity to do it, but so far the only belly i've painted is my own! Any tips for a first-timer? I've read that it's better to have mum standing at the beginning to map out the design? And do i need to use anything to seal the paint in the end? aarrggghhhh, i just love this crazy job!