Most mobile phones/cellphones have a rubbish proprietary adapter for which they supply some terrible headphones hard-wired into a handsfree kit.
What this instructable allows you to do is change those headphones into a headphone socket, so that you can plug your expensive headphones in and enjoy movies/music on your phone like it should sound.
A photo of the supplied adapter for my O2 Trion XDA phone is below. It has some terrible looking headphones that I haven't even bothered to try out because I know they'll suck.
Step 1: What you will need
You need the two items pictured below. The first is a handsfree kit which comes with your phone. It has a remote bit on it with a volume control and a microphone. It also has a Mini-USB plug, and some dreadful headphones.
The second picture is a 2.5" to 3.5" headphone socket adaptor off ebay which cost about 99p ($2). NOTE that there is a wire inbetween them - don't get an all-in-one adaptor.
You also need:
Soldering Iron + Solder