First I used a headphone adapter (1/8 to 1/4) just to attach the headphones meant for an mp3 player into the jack. This isn't really necessary to amplify the sound because the amplification is purely from the cords picking up the guitar's vibrations, but it was an easy way just to attach it on so the cord won't slip on. I'm sure duct tape would work just as well.
Then I looped the cord around the first strap clip-on a couple times, looped it around the back of the guitar, keeping it taut, wrapped it around the second clip-on and then put the earbuds into my ears. The most important part of all this is that the cord is taut through out, so it can pick of the vibrations. It should look something like this at the end:
Anyways, it worked really well for me on vacation, it gives it a lot more bass and gives it almost an acoustic sound to it.
EDIT: Image links fixed, my bad. I incorrectly assumed since it worked in the preview it was publicly viewable.
images resized, with love
