Before you commit to pouring concrete in your slab forms, you need to make sure that the slab is in "square." Being "in square" means that all corners are at 90-degree angles. No matter what shape your slab takes—whether rectangular, square or "L" to name a few—as long as your corners are in square, then your slab will considered correct. A squared slab is important because everything else that is a part of the building is constructed and placed according to the measurements of the slab. There is a simple old-school trick to how to square a concrete slab that can be used on any slab, no matter how big or small.
Select a corner at which to begin. Measure from the inside face of the corner down the length of the form that joins it and place a mark at a spot three feet from the corner. For example, if you have one form board that runs north and south that is joined by a board running east and west to form the corner, hold your tape to the north/south board and measure down the east/west one.
Measure (at the same corner) from the face of the east/west board along the face of the north/south form board that joins it and mark it at four feet from the corner.
Measure diagonally between the two marks. For your corner to be considered "in square," you should measure five feet. If you do not, move the corner by pulling the boards in one direction and checking the measurement. As you move the boards you will reshape the corner, keep moving the boards until the measurement is exactly five feet.
Repeat these steps for all of your corners. When all of them measure 3-4-5, go back once again and double check to make sure that none of your adjustments affected the measurements of another corner. When they all are at 3-4-5 then your slab is in square.