Total cost was around $55 not including tools. Each of the 4 shelves are 21"x 45" and the assembled structure stands 78" tall.
Step 1: Materials & Tools
10 * 8' 2 x 4's @ $2.59 each = $26
1 * 4' x 8' sheet of 1/2" thick plywood = $20-$30
80 * 2.5" exterior screws = $5 (just buy a 1 lb box)
1" finishing nails
Set of drill bits
Step 2: Cutting the Lumber
The plywood will go on to become the shelves, so it will need to be cut to dimensions of 21" x 45". You will need 3 of these sheets and one sheet of 21" x 48" for the top shelf.
4' x 8' plywood --> 3 * 21" x 45" plywood + 1* 21" x 48" plywood
Each shelf consists of two 4' boards with three 18" boards sandwiched between them. These shelves rest on four vertical 78" boards.
Four boards should be cut at 18" --> 4 * 78" boards + 4 * 18" boards
Four boards should be cut at 48" --> 8 * 48" boards
The remaining two boards should be cut into 18" sections until the total number of 18" sections is 12.
Step 3: Measure
Measuring is the most important step, so make sure you are accurate or else the shelf will not be level.
Each shelf height is up to you, but my suggested measurements can be seen in the image.
Step 4: Build a ladder
Pre-drill 2 holes with a drill bit only slightly smaller than the diameter of your screws. This is necessary to keep the wood from splitting when you insert the screws.
Screw the two pieces of wood together.
Wash, rinse, and repeat until you basically have a ladder, then build another ladder which will make up the other side of the shelf.
Step 5: Start on the shelves
Step 6: Putting it all together part 1
Lay both ladders on their sides with the 18" segments on the insides, then lay the cross-beams built in the last step across the ladders where the 18" segments attach.
Make sure everything is square, then start screwing everything together (after pre-drilling holes) with 2 screws per joint to prevent rotation.
Step 7: Putting it all together part 2
Make sure to secure the remaining boards to the cross-beams as well.
You may need straps to hold the sides in as you secure the final boards.
Step 8: Attaching plywood
Because this thing was so tall, I did the top shelf while it was laying down, then flipped it up to attach the remaining pieces of plywood.
REMEMBER: the top piece of plywood is the largest piece because it over-hangs the supporting studs.
Additionally, once you raise your shelves upright, make sure to check to make sure they are level,