Knock temporary stakes in place with a mallet and tie strings between them to designate the outer dimensions of the hexagonal deck. Dig holes for the precast concrete piers at each place indicated by the plans -- at each of the hexagon’s 60-degree angles, as well as one in the center -- with a shovel or post-hole digger. The piers typically lie at 1 o’clock, 3 o’clock, 5 o’clock, 7 o’clock, 9 o’clock and 11 o’clock of the project, if viewed from above.
Pour gravel into the holes and top with a pier. Lay a 12-foot 2-by-4 or deck board across each of the piers and the central pier topped by a spirit level to check that the piers are level. Add or remove gravel to accurately level the piers.
Place a 12-foot joist per your architectural plans -- typically a 2-by-6, but it may be larger depending on design considerations -- in the cross grooves of two facing concrete piers and across the central pier. Use a precast pier without grooves in its top for the central pier, so that the ends of cut joists can meet the solid master joists. Cut additional joists -- provisionally around 6 feet long, if the entire deck is 12 feet across -- to connect from the other piers to the master joist, beveling their edges to fit and toenailing them with 10d nails.
Add additional 2-by-6 blocking to support the joists, running between the piers at 11 and 3 o’clock and between the piers at 9 o’clock and 5 o’clock, meeting at the joist running between 1 o’clock and 7 o’clock, as well as halfway in between the two, meeting at the central pier. Drive stakes into the ground beside each joist about 2 feet in from the perimeter to lend rigidity to the structure.
Cut, bevel and screw in place lengths of 2-by-6 lumber -- termed fascia, as they provide a facing for the underside of the deck -- connecting the joists from one pier to the next to outline the outer edge of the deck and provide support for the ends of deck boards.
Lay the first deck board so its edge touches the center of the master joist and so each end overhangs the fascia by at least 2 inches. Nail or screw the deck board in place along the master joist and where it crosses additional joists or blocking. Continue adding boards, spacing them the width of a 10d nail until the surface is complete. Mark an octagonal outline allowing for a 2-inch deck board overlap of the fascia on the surface of the deck boards. Snap a chalk line to mark the outline and saw off the board excess.