Start out with the 3:4:5 rule. Start at one corner you have to hold, measure about 15' (3x5) for the one side you want set in place. Then measure 20' (4x5) for the adjacent side. Move end corner so the diagonal connecting the two ends is 25' (5x5). This will give you a square.
T check any layout, measure the two diagonals between the corners. When they are equal, it is square.