Gather pieces of wood or metal for your shelf boards. Lengths do not need to be exact; varying lengths will create an interesting staggered look. If your pieces are dramatically different lengths, use a hand saw to cut them down to size. You can use a variety of items for shelf pieces: old wooden planks, metal cabinet doors, and even full-size wooden doors can all be used for sturdy shelving. If your items are lightweight, you can even use stiff cardboard for budget temporary shelving.
Be sure that the ends and corners of your shelf pieces are not rough, and sand off any jagged pieces. If you will be storing anything delicate or fragile on the shelves, consider sanding the surface of the wood or metal to remove pieces that can snag clothing or scratch valuables. For cardboard, cut off ends that are tearing.
Place supports between each level of shelving. The number and strength of supports will vary based on the materials you are using. For sturdy wood shelves, you can use supports on the outside ends of shelves. For cardboard and metal shelves, which may tend to bend easily, you will probably need to place a center reinforcement to ensure the integrity of the structure. Cheap support options include cinder blocks, which are strong enough to support heavy books; plastic crates; and sturdy cardboard boxes, which should only be used for cardboard shelving.
If you are making cardboard shelving, create extra support systems by reinforcing shelf materials with dowels, curtain rods, broomsticks or extra pieces of wooden stripping. Use duct tape to attach the reinforcements to the length of the shelves.
Add levels to your shelving unit as needed. If you have an artistic eye, consider using varying lengths of wood and different support positions to create a geometric pattern. Place supports and shelving with an eye toward stability and strength so that your belongings can be stored safely.