If you have a triangle with one 4-foot side, one 3-foot side and a hypotenuse (diagonal) of exactly 5 feet, then the angle opposite the hypotenuse will be exactly 90-degrees.
First, stake the location where you want the 90-degree angle and attach two string lines. Attach one of the two lines to a second stake, where you want the first side of the angle. Using a tape measure, mark 4 feet down this first string line. Measure 3 feet down the second string.
Place the tape measure on the 4-foot mark of the first string and then move the second string until it measures exactly 5 feet on the mark.
Stake the strings in place once the angle has been established.