Footers are concrete slabs placed in the ground underneath a building. The footers are placed beneath the frost line and provide support for the building's foundation while protecting it from the shifting that occurs from the freeze/thaw cycle. The footers are built by pouring concrete into wooden frames located in trenches inside the ground.
Consult your local zoning board to learn the depth of the frost line in your area and the building codes you must follow when constructing the footers. The building codes vary by municipality.
Spray paint the ground to mark the location of the trench for the footers. The footers should be at least as wide as two of the concrete blocks used for the foundation, but add about 6 inches to each side to allow for extra space inside the trench so you can work. Mark both the inner and outer edges of the footer onto the ground.
Excavate a trench between the two spray-painted lines that extends below the frost line. If the frost line is only a few inches deep, you can dig the trench with a shovel, but if it's deeper than that, use a 1.5-ton excavator.
Hammer wooden stakes into the ground in the bottom of the trench about 6 inches away from the trench walls.
Lay 2-by-4 boards on edge along the inside of the wooden stakes against the ground, and nail them to the stakes. Add a second row of two-by-fours on top of the first row, nailing them to the stakes.