Cut four pieces of two-by-four to a length of 96 inches, using a circular saw. Cut eight pieces of two-by-four to a length of 12 inches, and four pieces to a length of 56 inches. Cut another eight pieces of two-by-four to a length of 8 inches. Cut one end of each 56-inch-long piece at a 35-degree angle across its 4-inch-wide face.
Place two of the 56-inch-long pieces together with the cut angles facing each other to form a pointed arch. Screw an L-bracket onto both, to hold the pieces together. Twist a 6-inch-long screw diagonally through both pieces on the top of the arch, to reinforce the joint. Repeat with the other 56-inch-long pieces, to form a second arch.
Screw one 12-inch-long two-by-four between both ends of the two arches using two countersunk 4-inch-long screws per joint, to form an arch frame. Screw two more 12-inch-long pieces into the peak of the arch, using two countersunk 4-inch-long screws per joint. Screw two more 12-inch-long pieces into either side of the frame, equidistant from the top and bottom pieces.
Attach one 8-inch-long piece at either end of one of the 96-inch long pieces, using two countersunk 6-inch-long screws per piece. Screw two more 8-inch-long pieces onto the 96-inch-long piece, equidistant from the first two pieces. Screw a second 96-inch-long piece to the opposite end of the 8-inch-long pieces, to form a side frame for your arbor. Repeat this process to form the other side frame.
Lay one side of the arbor arch flat on the ground. Screw one frame into either side of the arch using two lag bolts per side. Stand the arbor in the desired position on your deck. Screw the arbor to the deck using two lag bolts through each of the 8-inch-long pieces on the bottoms of the side frames.