Place the batter boards beyond the corners of your planned area. Tie strings to them marking the outer edges of your deck. Next to the house, drive stakes as close to the house as possible and tie strings to them.
Check that the strings are close to level. The layout is square when the diagonal measurements of the area are equal.
The post layout depends on the size of the beams, local building codes and your deck design. Measuring from the house, locate the post lines with additional batter boards and strings. Mark the post positions on the strings.
Conway Woodworking 