How to Use the 3 4 5 Rule to Build Square Corners
To build square corners with the 3-4-5 rule, first measure 3 units from the corner on 1 side. Turn in a perpendicular direction from the first line and measure 4 units. Then, measure the diagonal between the ends of your 2 lines. If it measures 5 units, your corner is square. If it's less than 5 units, your corner is less than 90 degrees; if it's greater than 5 units, your corner angle is too large.
Understand the 3-4-5 method. If a triangle has sides measuring 3, 4, and 5 feet (or any other unit), it must be a right triangle with a 90º angle between the short sides. If you can "find" this triangle in your corner, you know the corner is square. This is based on the Pythagorean Theorem from geometry: A + B = C for a right triangle. C is the longest side (hypotenuse) and A and B are the two shorter "legs." 3-4-5 is a very convenient measurement to check because of the low, whole numbers. The math checks out: 3 + 4 = 9 + 16 = 25 = 5. Measure three units from the corner along one side. You can use feet, meters, or any other unit. Draw a mark at the end of three units. You can multiply each number by the same amount and still use this. Try 30-40-50 centimeters if using the metric system. For a large room, use 6-8-10 or 9-12-15 feet or meters. Measure four units along the other side. Using the same unit, measure along the second side, at – hopefully – a 90º angle to the first. Mark this point at four units. Measure the distance between your marks. If the distance is 5 units, your corner is square. If the distance is less than 5 units, your corner is less than 90º. Move the sides apart. If the distance is over 5 units, your corner has a measurement of more than 90º. Bring the sides closer together. You can use a framing square as a guide when you do this. Once you've got a square corner, you can check the room's other three corners to ensure they're the same. 
Understand the 3-4-5 method. Measure three units from the corner along one side. Measure four units along the other side. Measure the distance between your marks. 