well, in general, you should have right, left, and mic, with a ground for each on the iphone end, and all 3 will have a common ground. for the headphones, right and left, with a ground for each, and ground is shared.
you will want to remove the protective coating with a microtorch or other heat source, solder and heatshrink, as opposed to twist together. You will also want to look up the pinout for the iphone side of it, and use a continuity tester to identify what each cable usually is.
red is USUALLY right, blue or green is is usually left - the white on the sennheiser is unusual (i double check this either by following the cable to the end, or using a temporary connection with crocodile clips between a prepared, tinned end of the headphone cable and jack).
Red to red, white to green, and 'copper' to 'copper' should do the trick. all 3 have the pesky coating, so you'll need to remove it.
this is the exact process/tools that have worked for me in the past - but instead of the jack, tin both ends, press to check, then solder.
Best Answer
Pinouts.ru has a great description of the connector.
The iPodLinux wiki has a great page describing the serial protocol.
Here is the connector diagram (with the blank side of the connector facing up):
Pins 12 and 13 are Tx and Rx, respectively. Rx and Tx are TTL level. Pin 1 is GND, and pin 21 is serial enable. To enable the serial port, tie a 6.8 kΩ resistor to GND.