Very very easy and simple, takes about two seconds but it works very well.
Step 1: Materials :
Old broken headphones,
Old guitar lead (to connect to Amp / Old jack plug, larger than 3.5mm),
Step 2: Cutting...
Cutt the old headphones away and just have the lead, no speakers.
And cut the guitar lead in half, or to whatever size you need/want it
Step 3: Connecting
Strip about 1" of rubber away from the end of the wires, so you have the bear wire...
On the guitar lead kinda, untwist it so you have two, rather thick metal wires, and on the small jack plug lead you should have 4 wires, one green, one red, and two 'normal' copperish/gold colour.
Connect the two normal wires to the two on the guitar lead (i just twisted them lol)
Then cover each of the connections (both of themm!!) with duct tape so they dont touch, then go round both of them with more duct tape so now it is like one wire, with a little bit of duct tape on.
Plug nd play:D
Simple but very good, better than using a homemade amp