I installed a different extension first that came well recommended, but I just could not work out how to use it and instructions were poor.
So I tried the Dione Form Wizard instead. The trial version gives just enough of a trial to know whether it's for you or not. It definitely fitted the bill for me and I had my form designed and online in no time at all.
Very happy with it and well worth $19. It has saved me heaps of time and I have a smart professional looking form on my website now.
I'm no coder but quite experienced at finding my way round Joomla. This extension is so intuitive that after I watched the video demos, I did not need to refer to any further instructions.