Lay two 4-foot 2-by-4-inch boards parallel to each other on the floor. Place the broad side of the boards against the floor and space the boards about 4 feet apart.
Lay the 4-foot 2-by-4-inch boards on top of four 2-by-4-inch 6-foot boards, arranging them like the rungs on a very wide ladder. Line up the ends of the 4-foot boards with the outside edges of the 6-foot boards and space the 4-foot boards so they're 2, 4 and 6 feet from one end of the 6-foot boards. Nail the boards together where they overlap with a hammer and 10d nails.
Repeat the process described in Steps 1 and 2 to build a second structure like the first.
Place the two structures you've built upright and position them so they are parallel, with the 4-foot crosspieces on the insides, facing each other. Space the two structures apart by about a foot and slide 4-foot 1-by-12-inch boards between so the edges rest on top of the edges of the 4-foot boards. Nail the shelves onto the 4-foot boards.
Place the shelves in the position you'd like them to be.