Before you pour the concrete for your garage slab, you need to check and make sure that the forms are "in square." Being in square means that all the corners in the pour are at 90-degree angles. When you square up a concrete slab for a garage, you are checking that the corners are at the correct angle through a series of measurements. Even if you have a garage shaped like a rectangle, this method applies.
Start at one corner of the slab forms and measure from the inside face of one of the forms down the intersecting slab for 4 feet, and place a mark on the top of the form with a lumber crayon. Now measure from the inside face of the form you just marked down the intersecting form 3 feet, and place a mark. Hold your measuring tape on the 4 foot mark and measure diagonally to the 3 foot mark. You should come up with exactly 5 feet as your measurement for this corner to be considered in square. If it is not, push or pull one form away from the corner (it will move the corner as well) and take the measurement again. Keep doing this until your measurements are the ratio 3-4-5. Do this for each corner.
Measure from one corner to the corner diagonally opposite it. Hold the tape to the very outside point of the corner when taking this measurement. Then measure the diagonal between the two remaining corners. These measurements should be exactly the same. If they are not then go back to Step 1 and double check the square of your corners because one of them is off.
Place stakes into the ground flush to the outside face of your concrete slab forms. Drive the stakes so the top of the stake is flush with the top of the form and then nail them to the form. Make sure you drive the nails in from the outside, this will make it easier to remove the stakes later.