Excavate the hillside with a backhoe or other digging equipment. Make the excavation slightly larger than your planned shelter, allowing at least 12 inches of extra space on all sides.
Lay a bed of gravel 6 to 8 inches deep across the entire excavation. Rake it smooth, checking it with a level to get it as even as possible. Build the form for the floor of your shelter atop the gravel. Use two-by-fours, securing them in place with stakes every 2 feet around the perimeter. Check your form to be certain it is square and level as you stake each side.
Pour the flooring of the shelter. Add re-bar or a screen mesh to the concrete for added strength. Use a trowel or a straight edge to smooth the concrete. Let the concrete dry for at least 24 hours.
Lay concrete blocks around the base of the flooring, leaving an small opening for a doorway. Cement each brick in place, keeping the row level and straight. Add additional rows until the wall is approximately 3 feet high. Allow the concrete to set for 24 hours.
Pour additional gravel around the three sides of the shelter that will be concealed in the hillside. Lay approximately 8 inches of gravel, then install 4-inch, perforated PVC pipe in a continuous run around the shelter. Add 3 to 5 inches of gravel--enough to cover the pipe. This system will help draw water away from the shelter and keep the interior dry.
Install additional rows of concrete blocks until the shelter is the desired height. Atop this wall, construct a frame for your roof by laying a sheet of 3/4-sheet plywood across the opening and building a frame around the exterior of the plywood with two-by-fours. If you're constructing a large shelter, add bracing inside the shelter to support the ceiling until the concrete is dried. Measure and cut four-by-four beams to wedge beneath the plywood. Two or three supports should be adequate for most structures.
Pour the concrete roof/ceiling of the shelter. Once the concrete has dried, back-fill the areas around the shelter, covering as much of the shelter as possible with soil. Install a solid doorway, preferable with a metal door. Add electrical or phone lines if desired.