Mark the four corners of your slab using wood stakes and a hammer. You can find the start point by looking at the overview page of your permit drawings and getting the distance from the edge of lot point (the establishing point of the property) to the starting corner of the slab. If your project does not have this marked on your permit drawings(as is the case in most residential projects), look for the surveyor's marker. This is usually a stake with an orange ribbon that marks the outside of the property, on a piece of paper, figure out how close your furthest corner has to be to the edge of property to allow for the slab. Measure each corner from the starting stake, just walk in what you think is a straight line the exact amount of feet and drive in a stake, you'll check the measurements in the next step.
Hold a tape on the center of one stake and measure the distance to the stake diagonal to it. Now, go to the opposite corners of your slab and measure that distance. If they are not the same, adjust one corner's stake only until the measurements are the same. This will put your stake's roughly in square.
Drive a stake into the ground next to each of your four corner stakes that is 1" outside of the slab. Tie the end of your Mason's line to one of the outside stakes and run the string to all four of the stakes you just put in the ground. When you are done, you will have a square outlined by string with four stakes, the "true corners" of the slab, set inside.
Lay your 2x6 boards inside the string until you have enough to frame in the entire slab. Cut the boards if necessary to make them fit. Remember, you want the boards to run from string to string.
Hammer a stake into the ground every six feet around the square on the outside edge of the string. You want them to be firmly in the dirt, but leave at least 4 inches sticking out of the ground. You will be nailing your boards to these stakes.
Stand each board up on the inside of the string line, starting in one corner with the end of the board touching the corner formed by the string with the bottom edge in the dirt. Drive a nail at the top and bottom of each stake. No more, you will be adjusting the form boards and do not want them locked in. Make sure the boards butt each other and leave no gaps.
Measure the diagonals and adjust the corners till the distances are equal. Look down the string line each time to make sure that the whole line of board is still straight.
Measure 3' on one board from the corner out. Now, go to the opposite board in the same corner and put a mark that is 4' from the corner. Measure the diagonal between your two marks, it should read 5'. If it does not, your corner is not a perfect right angle. Adjust the corner until the measurements are 3', 4' and 5'. Do this on each corner. Check the overall diagonal measurements again. Check that the sides are still straight and following the string line. When you are satisfied, go on to the next step.
Place a stake every 16" on the outside of the boards. Nail the boards to the stakes using a top, center and bottom nail. Use a level to make sure your boards are straight up and down. Do this around the whole frame of the slab.
Using a shovel, pile dirt against the outside of your form. Don't skimp on the amount of dirt you use, the average concrete slab pour will put 50 to 75 pounds per square inch pressure on each foot of board. You do not want to risk your frame moving out of square.