I once had to build a half circle deck. Cutting the deck boards was not the problem, the skirt board which was two 16 foot 2x6s had to be bent into a perfect half circle.
What I did was to lay out the 2x6 and on the BACK side, make straight lines every one inch all the way down the board.
Then with my circular saw, set the dept to about one third the thickness of the board and make the cuts on each line down the board.
The kerf cuts will allow the board to bend in on the cut side to your design. Be very careful when nailing as you might split the board. I predrilled all my holes and used screws only.