First decide the placement of the deck. You can build a free-standing deck or one that is attached to the side of your house. If the deck is adjacent to a house, it should be about 1-inch below the interior floor or a comfortable step down, usually about 7 inches.
Mark the deck corners with stakes and string. Check the squareness of the projections by measuring between stakes. Then measure diagonally between the corners. If the two diagonal measurements are equal, the corners of the deck are properly marked.
Place the posts in position. If you are using a ledger on the side of the house, you will need three posts. For a free-standing deck, you will need six. Post lengths will vary according to the contour of the ground. Establish the at one corner and use this to measure the others. Accurate measurement and trimming of the posts can be achieved using a string level or a carpenter's level. Post tops should be 2-inches below the top of the skirt boards. This provides room for the deck boards. Toenail posts to the nailing blocks of precast concrete footings.
Concentrated loads of planters, spas and other heavy objects may require additional joists and larger beams for your deck. Local building codes should be consulted regarding structural regulations. Design loading, with deflection limited to L/180, sets a maximum two-foot span for 2-inch decking. Provide 1/8 inch minimum space between boards, and center all large knots and butt-jointed board ends over joists. Beam span is the distance a beam extends from one post to the next. Beam spacing is the distance between beams. Deflection is limited to L/240.