Curved decks are interesting and unique but somewhat difficult to build because deck framing materials are straight and rigid. Start with a good plan. Curved decks are easier to draw on paper than to build in the real world. You will need to use a series of angled dropped beams, posts, and footings to support cantilevered joists that will extend across the curved or round section of arc. Joists are usually allowed to cantilever a beam by 2' without hiring an engineer. You can run the ends of the joists wild past the beam because you will be cutting them back all at once. Next lay out and mark the radius across the ends of your joists. You can use a trammel which is a piece of wood that acts like a large compass. Use your plan to locate the pivot position and fix your trammel to the center of the deck. Arc the trammel across the front ends of the joists to mark the radius cut with a construction pencil. Cut the joists to the proper lengths and prepare your front rim board to cover the ends of the joists against the curved section of deck.
The rim board can be bent using kerfed cuts. This involves scoring the 2x material with a circular saw to allow it to bend across the radius. Or you can use 1/2" plywood in layers built up to 1.5" thinkness.
Another method for installing a curved rim or header board is to soak strips of pressure treated plywood in hot water for about an hour to increase its flexibility. You can then bend the pressure treated plywood strips across the front of the joists and screw it into place.
To finish a curved deck you will need to install blocking along the curved ends of the deck to support the decking. Composite decking
is flexible enough to be bent across curved sections of deck. If the decking is hot it will be even easier to bend into the desired shapes. Fascia board is flexible and can be installed over the rim board to clean up the edges. Some composite and metal railing companies offer curved railing that can be custom ordered to match your decks radius.