To add garage footing detail, dig trenches for footings around the garage to the local building code's minimum depth and width. Install wooden boards in the trenches, and hold them in place with wooden stakes. Rebar the footings with rebar chairs, pour concrete into the footings, and install a rebar at equal distances to improve durability of footings.
Footing depth depends on local building codes, which typically state that they should be between 12 and 18 inches wide and at least 18 inches deep. A minimum depth of 48 inches below ground level is required in areas where the ground freezes so that the footings get beneath the frost line and the concrete stays in place. Dig a trench according to local codes, and remove around 3 or 4 inches of soil in the trench before setting the concrete pad.
If water may pool in the footing trench, use large cobbles at the bottom, and compact them into the mud to provide bearing. Set the rebars into the trench, and use rebar chairs to hold them off the sides and bottom of the trench. Cast the footings with a stiff concrete mix, and space out rebars every 12 to 16 inches along the footings before the concrete dries to tie them with the garage's foundation walls.