Dig the trenches for the footer to be laid in at exactly the right depth that you need. The ground that you are building on should be solid, stable and away from any flooding hazards. Keep footers at a constant depth. Do not lay an uneven footer to account for changes in slope. Make the trenches fit the gradient, not the footers.
Build the wooden frames with an exact thickness in mind. It's easier to level the final footer if your frames are level. Check to see that they are level before and during the pouring.
Use your laser level or transit to check that the trenches are level before you pour the concrete. You can also place stakes or sections of rebar (reinforcing bar) in the ground and place a mark uniformly on each stake to indicate the exact height to which the concrete should come. The laser level or transit should be used again to make these marks.
Place any rebar or other reinforcing material in the trench again at a constant level. The more level that each part of the process is, the more secure your structure will be in the long run. Rebar should be secured and kept even with cradles and wire ties to withstand the weight of the concrete.
Pour the concrete and smooth to level with trowels. Constantly check your level markers on your stakes and adjust accordingly. Once dry, use a hand level to check individual sections that you feel may not be level. If you discover a section of concrete that is not level, contact a construction professional to have it examined and see if there is a solution based on the size of the building to be placed on it.
Contact an inspector to ensure that your footer meets the building requirements of your area before you begin to build to protect you from liability.