Select two of the attic rafters to build your shelves between. Measure from the floor to a spot 2 inches up the rafters. Cut two 2-by-4 boards to that length. These will be the front supports of the shelves.
Place the two boards vertically, with one end on the attic floor and the other end against the side of the rafters. Nail the boards to the rafters.
Measure 12 inches behind the set of front supports and again measure up from the floor to the rafters. Cut two boards to this length and nail them to the rafters, 12 inches behind the first set. These are the back supports.
Cut a pair of 16-inch 2-by-4 boards for each shelf you want to add. Nail these as crosspieces, connecting the front and rear supports at the same height on both sets of supports.
Measure the distance between the crosspieces of the right and left supports, and add 4 to 6 inches. Cut a 1-by-12 board to match that figure. Slide the board onto the crosspieces and nail it to the crosspieces where they meet.