This is how to make a adjustable belt out of paracord (other rope will work too, but you don't want really thick rope..) using a Prussik knot as the buckle.

A little definition:
A Prussik knot is tied onto another larger rope and can be slid up and down the rope.
When the 'tail' is pulled, it will attach itself to the rope.
Pairs of these can be used to climb up ropes easily.

Step 1: What You Need - Skill and a Rope

Materials:
Paracord - The one pictured below is 2 metres, but this changes depending on your belly circumference.
Karabiner - Optional, good for holding the end of the rope, and tying the prussik around
Skills:
How to make a prussik - Next step.

Step 2: Tying the Prussik

1. Make a loop, fold it over (Picture 1) Put your finger through the holes, it should become picture 2.
2. Pick up the centre bit, and fold it over (Pictures 3 & 4)
3. Feed the rest of the cord through this (Picture 5)

It will pay to practice this a bit, until you can do it easily.

Step 3: Make It Resemble a Belt!

First, fold the rope over, and tie an overhand knot (as in the picture..)

This is how much the knot can be adjusted - just like the amount of holes on a normal belt...

Step 4: Bring the Two Together!

If you haven't got a prussik in the end of the rope, tie one now, it should be at the opposite end to the loop.

Feed the loop (on one end) through the prussik (at the other)

Ta-Da! one nice, new belt. if it isn't big enough, try making the loop a bit smaller, to free up some rope. If this doesn't work, you'll have to get a longer piece of rope. Or lose some weight. Your choice.