I get these off-cuts from my local timber merchant. They're 20p (0.25 USD) each for about 25mm by 25mm by 1,200mm. That means this set of shelves - which I'll use as a drying rack - costs about £5 (6.5 USD) in wood, plus some glue, staples, coat hanger wire, and time.
Step 1: Lay a Shelf Out.
The shelves are going to be several horizontally laid off-cuts, with one off-cut width between them. I want my shelves 5 0ff-cuts deep, so I need to space those with 4 more - 9 laid down.
Line one end up straight, and clamp. The piece of wood on top will become part of this shelf, running front to back.
The other end needs cutting and sanding neatly. Put two pieces of wood on top. The outer one becomes part of your shelf, so measure that from the other end to set your width. The inner one is just there so I can clamp a piece of wood below our build a bit further in, leaving clearance for a jigsaw.
Make sure the odd numbered pieces are good samples. These are the ones we're keeping.
Step 2: Trim, Sand, Glue, and Staple the First End.
Cut the ends off, and sand. Remove the clamps, and place wood glue on every other piece - odd numbered only! Clamp again, and stable the front - again, odd numbered only! Cut the ends of the top, edge piece off. Stable the ends. You will still need clamps; the glue is wet.
Step 3: Glue and Staple the Second End.
As before, but you can skip the trimming if you lined your shelf up nicely.
Step 4: Remove the Spacers.
If you've only glued and stapled the odd numbered pieces, you should be able to push these out at one end with your fingers, supporting the ones you're keeping either side. Just wiggle them free.
If you've used your worst off-cuts as spacers, this is a good opportunity to see which ones broke. If they survive this, perhaps promote them to usable.
Step 5: Make Several More Shelves.
I recommend putting the radio on at this point.
Step 6: Cut the Legs and Brackets.
You will need as many brackets as there are shelves (six for me) multiplied by legs (normally four). These are 100mm long.
Cut your legs to length, and mark where the shelves will go.
Step 7: Construct the Legs.
Glue the brackets, hold them against the legs, and stable.
Step 8: Put the Shelves Up.
Congratulations - you have a flat pack! If you're like me, it's now too big for your workbench. Graduate to the floor.
Clamp one leg in place, then stand the shelves on one end, so the clamped leg is at the top. I used a long clamp at the base like an outrigger, to stop it from falling. This was the best position to begin stapling. You don't need to glue the shelves to the legs, because these staples don't take much weight.
The other three legs are simpler to add. Just be aware that your shelving unit has no rigidity at any point of this step.
Step 9: Add Rigidity.
Cut coat hanger wire to around 100mm long for corner brackets or 200mm long for stiffening back to front, if your shelves are a similar size to these.
They need bending into an ornate f shape, looping one way at the top and the other way at the bottom. This means you can screw clockwise with the bend, so it doesn't loosen when you fit them. You can bend them around the tip of some thin nosed pliers.
Screw them to your shelves. I recommend short 3mm screws, but I only had 4mm screws spare! You'll need washers.
Please comment if you think you have a better strategy for placing this wire, but I put them on alternate ends of each shelf, back only. I then added cross braces, back to front, at each side, towards the top.