The foundation is considered the "bones" of a building; as strong as the bones are will be how strong the building is. By adhering to building regulations and zoning laws you can build a garage foundation that is suited for your area's soil and climate. The actual process of building the foundation is simple, just painstaking and it requires a fair amount of physical labour.
Measure the approximate layout of the garage foundation and drive a survey stake into the ground at each corner. When all four stakes are in place, measure diagonally from corner stake to corner stake and adjust the stakes until each diagonal measurement is the same as the other. Your foundation will then be squared.
Tie mason's string between the stakes about 10 cm (4 inches) from the ground. The mason's string now marks the outside edge of your walls. Measure 15 cm (6 inches) out from that line (outside the building) and place a stake, then measure 30 cm (12 inches) from that line into the building and place a stake. Connect these stakes with mason's string. You should now be looking at the outline of three squares. The original line is still the outside of your walls, but the other two mark the width of your garage foundation.
Dig out the width of your foundation to the depth specified by your planning requirements -- in most areas it will be 90 cm (3 feet) in depth -- plus 15 cm (6 inches) So if your code calls for the foundation to be 90 cm (3 feet) in depth, dig a hole that is 105 cm (3 feet, 6 inches) deep.
Spray the foundation hole with water and use a hand tamp to tamp down the bottom soil so it is even and compact. Add 15 cm (6 inches) of gravel to the bottom of the hole and use the hand tamp to even out the surface of the gravel. The gravel provides water drainage for your garage foundation.
Place your concrete forms into the hole, making sure they are level and supported to withstand the force of the concrete. The type of form system you have rented will determine how best to support the forms.
Measure in from the outside edge of your wall (from the original stake and string) to locate the centre of your wall. Drive a nail in the edge of your forms to mark the centre and run string between them. Then, run a 5 x 10 cm (2 x 4 inch) board under the string the entire length of your foundation. Drill holes to allow you to hang your anchor bolts for your sill plate after the concrete has been poured (but is still wet).
If rebar is required by your local planning department, pre-tie or float into the wet concrete a small rebar support mat -- three lengthwise pieces joined by a short cross piece every 1.2 m (four feet) -- and place in the hole before you pour.
Set some corner stakes that are 60 cm (2 feet) offset from your original outside corners so they will not be disturbed by your digging your garage foundation; this way you will not have to remeasure when it comes time to find the centre of the wall but will have an offset stake in place.
Do not build a garage foundation without the getting the proper planning permission and checking the building regulations. Permits are about more than taxes and paperwork, they are to ensure that according to the soil type of the area that buildings are constructed safely. You do not want your finished garage to collapse in a rainstorm because you didn't know how deep the foundation needs to be in your area.