For this instructable you'll need:
* A knife capable of cutting cardboard
* A rectangular cardboard box you don't need for anything else (such as that box where your last internet order came in)
I assume you know that knives can be sharp and give nasty cuts if used improperly and that you know to be careful when using them. That being said, let's get started!
Step 1: Mini-shelf: removing the flaps from the box
Cut through the fold between the flap and the side of the box. Pay attention on how you cut because it will define what the front of your mini-shelf will look like. Repeat this for the other flaps and keep the opposite flaps together.
Step 2: Mini-shelf: Preparing the shelves
Do the same for the other flaps that will be your vertical supports. The slits here will determine the height of your shelves, so again, think this through! When you mark the slits, remember to put them on the opposite side from where you marked them on the shelf flaps. This is so you can have all the straight edges on the front. In this project I put the slits for the shelf flaps from the straight edge to the cut edge, on the vertical flaps I had put them from the cut edge to the straight edge.
A note on the making the slits: the slits will only provide stability to the construction when the slit is not wider than the cardboard that goes into it. When it is too narrow however, you may experience difficulties during assembly. If this is the case, make the slit a bit wider. If you made it too wide, either put some tape in the corner after assembly or try to make it narrower by applying tape on the inside of the slit until it is narrow enough.
Another thing to take into account when determining the placement of the vertical supports: don't space them too far out. When you place relatively heavy objects on the shelf it may bend or fold when the spacing is too far. The closer the supports are to one another, the more weight the shelf can support between those supports.
Step 3: Mini-shelf: Assembling the shelves
When this is done, your construction should be self-supporting.
Step 4: Mini-shelf: final assembly
To make things sturdier you can apply tape to the joints.