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.
The proper way to do this is to strip back the insulation and twist the two ends together, then solder (weld) and cover in electrical tape.
Without equipment to solder, your next best bet is a butt splice. There are many different kinds of butt splices; this is just an example of a very basic and common one:
.
You strip back approximately an inch of insulation on each side, insert them into opposite ends of the splice, then use crimping pliers to crimp the splice on the wire for a tight connection. If you don't have crimping pliers you can use a set of Linesman's pliers, although the crimp won't be as good. I usually use black electrical tape extending about an inch on either side of the splice to help ensure the connection doesn't get snagged on anything in the future.
The best way really is to solder it, and a soldering iron isn't more than $15 with enough solder to last you for a few years if you end up doing this kind of thing only occasionally.
Best Answer
I would solder the wires together and then use heatshrink tubing to cover the joint and help make things look neat and tidy.
Not sure how much you know, so I'll just keep going...
Solder is made to join electrical contacts, and it makes a durable bond. You need a soldering iron to heat solder. The basic approach is to hold the iron on the two (or more) wires to join, then apply solder to the heated joint. The solder will melt quickly and harden even faster after you take the iron off. That's a super basic description, and there's a ton of soldering tutorials around the web you can look to if you need help.
Heatshrink sounds like what you want if electric tape is "too messy". You basically cover the joined wires with it, then use hot air (via hot-air rework station, or just a hair drier) to shrink the tubing around the wires.