Set a builder's or laser level on a tripod or table at the garage's opening to ensure the area is level.
Pound a 1-inch-by-4-inch 2-foot long wood stake into the ground at the back corner of the garage. Pound stakes in the ground flush to the walls every 4 feet with a mallet or sledgehammer until you reach the front of the garage.
Place a mark on one of the stakes to indicate how high you want to pour the garage floor. Set a 6-foot stick or leveling rod vertical to the marked stake with its bottom resting on the ground.
Transfer a matching mark from the stake to the stick to indicate the pour elevation. Mark another line where the laser or level site's horizontal crosshair is aimed.
Move the long stick to each stake in the ground and line the top line up with the level. Find the stick's bottom mark and transfer it to the stake you are lined up with. Repeat this process with each stake.
Line the top edge of an 8-foot 1-inch-by-4-inch piece of lumber to the marked elevation lines on the stake. Fasten the board into place using 8d nails and a hammer or 2-inch long screws and a screw gun.
Place another row of stakes and wood rails every 8 feet across the entire garage floor. Check each of the stakes with the level after securing the elevation rails.
Pour concrete using a cement truck with a spout, or wheelbarrow it into the top of the wood rails starting at the back of each 8-foot divided section and working your way to the front. Place the 9-foot screed on top of the rails and work in a back and forth motion to level the concrete flush to the top of the wood rails.
Remove each set of elevation rails and stakes as you work your way across the garage floor. Shovel concrete in the sections where the stakes and rail were, and trowel the area to smooth out the indentations.