Cut 8 stakes 600 (2ft) long, out of 50x50 (2x2) stock. Cut a point at one end of each stake so they can be easily hammered into the ground.
Position the stakes as shown in the diagram and hammer them firmly into the ground.
The stakes should be at least 600 (2ft) away from the building line to allow room to dig the corner footing holes.
Make a level mark on all eight stakes beginning approx 150 (6") above ground level, at the corner where the ground is the highest (if the ground is sloping).
Use a long straight edge and a carpenters level to transfer the level mark from stake to stake.
Once a level line has been established and marked on all eight stakes, proceed nailing 100x25 (1x4) batter boards to the stakes, so that the top of the batter boards are flush with the level lines marked on the stakes.
Run a taut string line from batter board to batter board, passing directly over the pegs and tie the ends to nails hammered half way in to the tops of the batter boards.
This will now show the approximate building line and perimeter of the pergola, but more exact measurements are needed.
Check that the distance between the string line at point AA and BB and the distance between DD and CC are equal. If not, make any necessary adjustments by moving the string line along the batter board and adjust the nails accordingly. The distance between AA and CC and the distance between BB and DD, also need to be equal. Make any necessary adjustments.
Once the perimeters are parallel, the diagonals need to be measured to ensure that the building line is square.
Do this by measuring the distance between AA and DD and the distance between BB and CC (the diagonals). Make any necessary adjustments to ensure the diagonals are equal and if adjustments are required, re-check the parallels again, since altering the diagonals will also change the parallels.
When the building lines are parallel and the diagonals are equal, the building line is then square. You now have a level, square building line to work from.