Screw together the 20-inch-long 1-by-3 pieces to create L-shaped assemblies.
Cut the ends of the assemblies so that they will fit against the underside of your roof when they are placed against a rafter with one leg horizontal and the other vertical. If you have a 45-degree roof, make a 45-degree cut.
Screw one of the assemblies to every other rafter at the height where you want the shelf. If you want two shelves, you can make more L-shaped assemblies and screw two of them onto each rafter, one above the other.
Measure the distance from the inside of the vertical member to the outside of the rafter behind it. Cut a piece of 3/4-inch plywood to this width.
Slide the piece of plywood in between the rafters and the vertical members of the L-shaped assemblies so that it is resting on top of the horizontal members. You now have a sturdy attic shelf for storage.