Make a simple, handy, solid box that's just the right size for what you want to hold. This cardboard box is perfect for gifts or handy storage; I love to put collections of similar items in this kind of box. Required tools and techniques are simple. These instructions are error-tolerant and adjustable to whatever size you need but if you get much smaller than i have the tolerances will get tighter.

Supplies

Cardboard -- I'm using junk corrugated cardboard from a shipping box. You'll need two pieces maybe 4x the size of your box and a strip about 4x as long as it. I started with a piece about 12" by 24" (15cm x 30cm) and had some left over.
Glue
Masking tape or similar
Blade / scissors
Ruler -- and a long straight edge if your box is going to be much longer than you can measure

Step 1: Measure First

How big do you want the inside of the box to be? I want about 12cm by 8cm, by 6cm deep so let's call those measurements A, B and C. You can use 12 inches by 860mm or whatever but for these instructions let A be as long or longer than B. Then measure from a straight and square corner of your piece of cardboard as i have here; C, A and C along one edge and C, B and C along the other edge. (In other words a strip of width C along two edges, then a rectangle A by B and then two more C-strips.) Measure the corresponding points on the far edges and draw lines between your measurements.

Step 2: Then Cut

Cut along the outside lines, then cut notches between the long sides and the middle measurements so that each short edge has two square flaps. Then, for maximum precision cut through one side of the inside lines that with just the tip of your blade or just the point of one of the scissors blades, without letting the it go all the way through. We're going to fold the sides up along these lines. For a box smaller than a few inches this scoring is important, the bigger your box gets (or the less you're worried about precision) the less important it is.

Pull off the inside layer of the cardboard and the corrugation from the flaps like in the 3rd picture. Again, this is for precision and you can skip it if you're making a giant box or you don't mind the box being sloppy.

Step 3: Glue the Bottom

Fold the sides up and glue the flaps on the small edges to the long edges. If you're dexterous enough you can do them two at a time. Make sure you line up the outside of the flap with the outside of the long edge; If your cuts and measurements are precise then if those line up everything else will be pretty square.

Step 4: Make the Inside of the Top

How deep do you want the top to be? How much of the bottom do you want it to cover? Call that length Z, i picked 4cm. Then measure a long strip of cardboard that's a little thinner than that, i measured a 3cm strip. This is our wiggle room. This doesn't have to be the same material that the rest of the box is made from; Using thinner cardboard or something similar will give you a sleeker box. I wouldn't use regular paper, that won't give enough wiggle room, and don't use something too valuable because we're going to cut and fold and fiddle with this and might have to replace it. It's only going to be visible inside the box.

Then set your box along the strip and mark out the long side twice and the short side twice. Cut the strip a little (5mm / 1/4") shorter than the total length you measured. This will be the inside of the top, we want to slide over the bottom snugly.

For the moment, tape the strip along the top of the long edge of the bottom of the box and fold it around (again i recommend scoring the outside and leaving the inside intact at the folds). Once you've got the folds right, disconnect the strip and tape the ends of the it together in a loop. I had to cut through the other long edge and tape it back together to get the fit i wanted. This piece isn't important for strength or looks so fudge it until it's the right shape.

Step 5: Measure the Top

At another good corner, make Z-width measurements along two edges. Then put your bottom with the strip around it at the corner of your two measurements and mark how big the top face of the box is going to be. It should be past the outside of the strip by a little less than the thickness of your material, so that everything we've made already will fit in the rectangle we're making. I came up with 13.5cm and 9cm. These measurements are our new X and Y, if you're writing them down.

Step 6: Cut and Glue the Top

Glue one of the long edges of the strip to one of the long edges of the top so that their outsides line up. Then glue the other long edge, and then the short edges and flaps. At each stage it's a good idea to fit the top-in-progress with the bottom to make sure everything's square and snug. If everything we've done so far has been precise that shouldn't be too much problem.

If something does need to be adjusted there are a few things we can do. Beveling the inside of the top or the outside of the bottom buy drawing something stiff along them will make fitting the top easier if they're tight just at the lip. If there's tightness over a big part of a face you can press the cardboard together, flattening the corrugation. And if anything's too loose you can line the inside of the top or the outside of the bottom with a bit of card, maybe some of the bits you cut off the flap or a piece of heavy paper or cereal box.

Otherwise you're ready to go. You can label it, paint it, line the inside, glue stuff to the outside or whatever you like. I'm going to draw on mine with glow-in-the-dark paint because i'm from the '90s.