To square a wall it can be done laying on the floor or with the wall standing up. This article will cover the basics for both ways.
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The basic rule of square is 3-4-5, meaning 3" of [run] with 4" of [rise] has a 5" [diagonal] measurements, or you can check the square of a wall by taking the measurement from corner to corner on both sides to make sure the measurements are the same. Last a wall can be checked for square after it is standing by leveling the wall up.
Squaring a wall up that is laying on the floor is mostly done if sheeting is going on the wall. Start by chalking a line on the floor to where the inside of the wall will be placed after its standing up. [standard 2x4 wall with 1/2" sheeting would need a 4" line] Nail the bottom outside edge of the bottom-plate to the chalk line, [it is to also help make sure your plate is running straight] Last take thw measurement from corner to corner adjusting the top plate to make the wall [in square]
If the wall is already standing and it needs to be squared up you would use a level on the inside and or outside corner of the wall that needs to be in square. If this is being done before any wall finishing is on the walls, all the wall that get leveled/squared should be braced from the top plate across the wall to the bottom.