Use a measuring tape to measure the size of your garage and mark its boundaries. Put wooden stakes on all corners of the garage, connect the stakes with the help of string as if to form a rectangle. The height of the string should be about 6 inches from the ground.
Spray orange colour beneath the string. Orange is usually used as it is the most visible colour which can be seen on soil or in grass, even in broad daylight. Measure the width of the foot from the base of the string and mark another line about 6 inches wide of the previous one. You will end up with two parallel orange lines on the soil, the gap between them is the breadth of the footing.
Dig the soil out of the footing area. You will require a pick axe and a shovel for this part if you intend to do it with your bare hands. However, an easier alternate is to use a machine excavator to dig out the soil. You should only dig the soil between the two orange soils such that the orange lines are still visible on the upper surface. Dig as deep as you can till you reach the frost line beneath the surface. If you do not know the depth of the frost line, call the local authorities to find out the depth of the frost line.
It is typically 3 to 5 feet deep. On reaching the line, compress the surface of the soil by jumping on it with your feet and your footer is good to go.