This isn't original as such, I learned it back when I was into origami as a child, but it took me ages to realize how handy they are. It's not at all a complex fold, so it should be ok even for people not generally into that sort of thing. I use them continually especially for a quick container to hold screws, bolts, small parts, etc when taking something apart and I'm somewhere where a more normal container isn't around but a piece of paper is.
Step 1: Start with a piece of paper
Doesn't have to be unused, in fact it helps a bit if it's printed with text for horizontal/vertical straight guides. In this guide I'll be using a half-letter sized promo flier for the movie Eragon someone sent me, now way past it's due date. Larger pieces make larger boxes and the shape has effects on the height of the sides, though anything rectangular (within reason) should work.