Consider excavating the pit for your underground bunker. It is recommended to make the pit at least 2 meters deeper than the bunker itself. Dig an area of your pit which should be deeper than where you intend to have your shelter.
Use surveying stakes to set the corners of the bunker and tie a piece of rope or string to make wall markers. Now consider digging trenches along the outside edge of the bunker walls. The trenches should be at least 1 meter deep and 1 meter wide. Use gravel to fill these trenches at least halfway. This will service drainage of your system, preventing the moisture from entering through the shelter.
Dig a shallow trench from the leach field to the slab. Consider running PVC pipe and extend it horizontally from the point where you to position the toilet. When you are done placing the pipe, consider refilling the leach field.
Place the 2 by 6 foot boards along the straight line. Shake the boards into the place upright to make a six inch deep box. Put the wire mesh in the box and add some concrete on the top so the box stays in its position. Smooth the concrete on the top by using a small piece of wood. Use a chalk to draw lines on the dry concrete floor. You can now see where the walls of your underground bunker will be. Mark where your bunker door will be placed and make a frame for it. You could use small pieces of wood from the slab to hold it in place.
Use plywood sheets across the roof joints and use a hammer to put them in place. Hang your door in the frame made from wood pieces from slab. Use a ladder to get down through the entrance of your bunker.