I want some decking that is a rectangular shape of approx 12ft x
12ft.

One of the corners however, I need to be rounded, approx radi of about
3 ft.

How do I set the beams underneath to support the decking thats on a
radi.

Also how do I finish off the radius'd edge. As normally a rectangular
piece of wood is used as an eding piece to a straight part of the
decking.

Any ideas approx how much in total a DIY job would cost me, and how
much a pro would charge.