To start off with you need:
Paracord - I used metres, but could easily be narrowed down to less.
Karabiner - Used as the 'buckle'
Pants to hold up.
Step 1: Make Loop for the Karabiner
The easiest thing to do is fold the rope over and tie a overhand knot.
It should look like so:
Step 2: Tie the Munter Hitch
Follow the three images. First a loop of rope is fed through the karabiner, then the free end of the loop goes back on itself.
Step 3: Tying It Off
Lie the 'tail' over the other end (picture 1) bring it under, and through the hole (picture 2) Pull it tight.
This should hold your trousers up. If it doesn't, undo the 'tying off knot' and tighten the Munter hitch, then tie it off again.