This is for using a woven band, or a strip of (preferably hemmed) fabric, or something along those lines, as a belt.
My current favorite knot for fastening a belt is a sheet bend. I've found that if I tie it carefully so that the belt lies flat in every step of tying the knot, that it is much less bumpy than a square knot. It is also easily adjustable.
When you tie a sheet bend in something that is round, it looks something like this:
(There are plenty of instructions online for tying the knot this way, if you want more information on that.)
But when you tie a sheet bend in something that is flat, you can make a knot that looks like this:
(It looks very similar, but one end of the knot was tied with it folded on top of itself, rather than sitting next to itself.)
Now I'll show you how I tie it. On the left, I've put the pictures in right side up, which is how it would look if you are tying the belt on somebody else. On the right, I've put the pictures in upside down, which is how it would look if you are tying the belt on yourself.
1. Wrap the belt around the person's waist, so that both ends are in front. Make sure that the belt isn't twisted anywhere.
2. Cross one end of the belt over the other end. The belt should be fairly snug at this point, but leave yourself however much room you need to work; you can easily tighten it later.
3. Take the end of the belt that is underneath and fold it over the other end.
4. Take the end of the belt that is inside the folded end, and carefully making sure not to twist it, loosely loop it over and all the way around both halves of the folded end. Your loop should end about at the same place that it began.
5. Take the end you were working with in the previous step, and bring it through the loop so that this end is on the opposite side of the knot than the end you folded in step 3.
6. Carefully tighten the knot so that the loop you made in steps 4 and 5 doesn't slip off the folded part of the knot, and so that it lies nice and flat.
To tighten or loosen the belt:
1. Pull on the folded part of the knot to get some slack into the knot. (You want to pull on the fold itself, not on the end of the belt that is attached to the fold.)
2. Adjust the part that goes around the waist so that it fits comfortably.
3. Pull on the end that was folded to tighten the knot back up.
You can also tighten or loosen the belt by adjusting the looped part of the knot, but I find the folded part of the knot to be easier to work with. :)