To make a root cellar, dig a hole in the ground in the desired size, and pour a concrete footer around the edge of the hole to support the walls. Use concrete blocks and mortar to form the cellar walls. Make an arched wooden form for the ceiling, set the form in place on the root cellar, and reinforce the ceiling with a rebar grid. Pour concrete into the form, allow the concrete to cure, and remove the form.
When building the walls, place a layer of mortar between each layer of brick, and allow the mortar to cure before adding the roof. Make the frame for the roof from arched pieces of plywood connected to a frame of 2 by 4 boards, and place sheets of plyboard over the frame before pouring the concrete.
To make the rebar grid, install 18 pieces of rebar in a grid-pattern with the center of each square in the grid 8 inches apart. Bend the ends of the rebar, and inset the rebar into the holes of the concrete blocks that form the cellar walls. The rebar should extend at least 1 foot underground. Use enough rebar to support the weight of the concrete roof.
After the concrete has cured, cover the cellar roof and walls with dirt, and install a door and stairs. In rainy climates, install a French drain outside at the floor level, and backfill the walls with gravel rather than dirt to ensure proper drainage.