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.