How to wire sennheiser headphones to an iphone headphone cable
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.