Compact the soil to be poured on. The dirt should be packed tight enough that it will absorb a minimum of moisture and provide support for the weight of the concrete and any equipment weight that may bear on the concrete. A whacker-packer works well for this but is gasoline operated so care must be taken if used in an enclosed area. Run the packer over the area until it no longer subsides under the machine.
Dig a trench around the perimeter of the area to be poured. The trench should be 8 to 10 inches wide or roughly the width of the shovel. The sides of the trench must be as near vertical (plumb) as possible and have a flat bottom and nice sharp corners top and bottom. Be careful not to step on the edge of the trench at any time and cave it in. Use the spade shovel to cut the trench and remove the bulk of material and then use the flat shovel to plumb the sides and flatten the bottom.
Flatten the surface of the soil. Using the level and the straight edge, check the floor for flatness and make any adjustments with the flat shovel. Remove material from high spots and deposit it in the lower spots until it reaches a good average. Stretch the nylon string across the work at the same level on both ends and use the tape measure to do a final check on the grade, or flatness, of the floor. Now is a good time for any pesticides to be applied to the soil. Spread the sheet plastic out to completely cover the surface to the inside edge of the trenches and trim off the excess plastic.
Cut the rebar with the bolt cutters to fit lengthwise and end-to-end down the trenches. The ends of the rebar should overlap a few inches to allow room for a steel tie. Lay the rebar into the trenches and tie the ends together. Rocks, chunks of concrete or bits of broken masonry blocks may be placed under the rebar to hold it up off the ground and help the concrete encapsulate it. Do not allow the rebar to contact the soil on the bottom or the sides of the trenches.
Roll out the steel mesh and cut it with bolt cutters to fit from trench to trench. Overlap strips of mesh by placing one mesh on each side. Position the mesh on top of the plastic and tie the mesh every few feet at the overlap on each edge with steel ties.
Line the outer edge of the floor with the form boards. If the floor is captive between walls, use the 1x6 form boards against the existing walls. If the work is not captive, heavier 2x6 form boards will be needed.