These shelves are held firmly in place by pressure created from the shelf itself, there are no screws, mounting hardware, or levitation tricks - all they need is two opposing walls, and they're good to go.  This simple shelving unit fits between any two walls.  In my case, a tiny closet in my room that was waisted storage space that I thought I'd improve.  The opening to the storage space is oddly smaller than the space itself (it's an old quirky house), so the fact they are pressure fit and self standing was actually a byproduct of the fact that I needed a method of installation where the shelf could be rotated into position, since the full width shelf wouldn't fit into place any other way.

The shelves are constructed from some simple dimensional lumber from Home Depot.  They're 1 3/4" x 1 3/4" and 66 inches tall.  The key piece to the design is the simple profile cut out that allows for the shelves to slip in from the top at an angle, as opposed to slid in from the front like a normal shelf - see the photos for detail, but simply put, it's a "G" shape.  I used a jigsaw to cut the notches.  The key measurement to consider is your overall width of the space you're filling with shelves since they're held in place by pressure alone.  Total width should be width of shelf + thickness of both supports (after notched cutout is made).  I cut my supports, and then adjusted my shelf width appropriately to get the best fit.