Step 1: Get Lumber
The wood will be measured in board feet. Roughly, a board foot is a measure of volume equal to 144 cubic inches; however, as with any trade specific measurement, it's not quite that simple. Rounding errors need to be dealt with properly.
Step 2: Straighten the lumber
Step 3: Rip the lumber to the same width
Step 4: Cut the pieces to length
Use a miter saw with a sharp blade. A blade in nearly any condition will cut pine from Home Depot, but hardwood is much easier to burn.
I like to hide the end grain on my shelves, so I cut the outside pieces on a 45 degree angle.
Step 5: Layout
Step 7: Pre-assembly
I use a set of tie-down clamps to avoid damaging the corners of the wood.