This is a fairly simple idea so the included photo's will provide most of the detail, but I know you can keep up.
Stuff to Gather
Empty envelope box (see next section for sourcing information)
Clear packing tape
Knife or scissors suitable for cutting cardboard
Pencil or pen
A sheet of lettersized paper (8.5"x11" or A4), or optionally a ruler.
Finding the Cardboard
Discarded envelope boxes are perfect for this task. The boxes you are looking for are those used to pack #9 and #10 letter sized envelopes. These hold 500 envelopes per box and are the standard size box for commercially printed envelopes in North America. Other countries should have something comparable. Best bet is to look in stationary storage at the office. See photo 2
On most boxes there will be a label on one end. This labeled side can be the top as it will be cut off and discarded. The closure flap of the box will become the back. See photo 3
First thing to do is mark our cut line on the front. This can be made at 4.25 inches or 105mm from the bottom. The quickest way to measure this is to fold a sheet of lettersized paper (8.5"x11" or A4) in half the long way. Then holding the folded sheet against the box sideways and aligned to the bottom draw your line. See photo 4 & 5
Next we need to mark off the section that we will cut from the top. Let's use the folded sheet once more, however this time fold the sheet in half again. If using a ruler mark approximatly 2 inches from the top. See photo 6
Now, let us make our diagonal lines. These are drawn from the ends of our first line to the back corner of our top line. For an optional and slightly refined final look you can make an additional mark on the top line 4.25 inches from the back and draw your diagonal to there. Either style you choose repeat this diagonal line on both sides. See photo 7 & 8 (photo 7 shows the diagonal being drawn to the optional marking)
Using your cutting device carefully cut along the lines we just made. Notice the word carefully? That's the extent of my safety message. See photo 9
The Final Touch
With the hardest part done simply place some tape over the back to secure the flap. At this stage you can tape any other edge that you feel will provide the most strength for what you plan to load this thing with. Get creative and decorate your finished organizer to suit the contents or your decor. If decorating you may want to place any reinforcing tape on the inside.
Having the box standing upright is perfect for magazines, booklets, or stapled printouts. With the box lying on it's side you can store file folders for easy access. I'm sure you will even find uses that I hadn't thought of. If so be sure to share your idea in the comments section.
Reuse the Refuse
In the last two photos you will notice that even the top section we cut off found a use. My daughter thought bed for her stuffed toy, my son thought robot shoes, and I thought small parts bin. Uses are apparently a plenty.
If enough people like this instructable I will post a video providing additional detail and bonus material. Simply give this instructable a rating to let me know what you think.