This is a 12 ft diameter cantelevered deck, with poured concrete piers. what follows assumes you will want to attach the deck to the house. Find a spot 6 feet from the edge of your house (plus an allowance for a ledger board) more or less centred on the house entrance. Mark this spot as the central support column of the deck. Drive a peg. Attach your string to this peg and, draw a chalk circle of radius 6 feet and a second circle of 4 feet. Start marking the position of each of the 8 support post. String a line to the right edge of your door and driving a peg at the intersection of this line and the 4 foot circle. Carefully counterclockwise rotate the string 45 degrees. Strike this second chalk line and hope the subtended angle covers the door entrance. If it does, drive a peg at the 4 foot intersect. If it doesn't, fiddle with these two lines so that the45 degree arc is centred on the door frame. After these first 2 pegs are in, progess around the circle until all post holes are pegged.