Here I will show you that its not so hard to do this hack. All you will need is an old ATM or credit card, and a soldering iron.
Step 1: Use ATM Card to Pry Open the Sennheiser Headphones
Use the ATM or credit card to push out the metal rings around the Sennheiser headphones. Be assertive and gentle when doing this, and do not use a knife or anything metallic, as you might accidentally damage or scratch your new headphones.
Step 2: Pop Open the Sennheiser Headphones
Step 3: Break Open the IPhone Headphone
Instead of cutting the iPhone cables, its best to use the complete length of wire, so I use a knife to break open the one functional iPhone headphone. Also it might be a good idea to mark the left and right headphone cable for later.
Step 4: Desolder the Cables From the IPhone Headphones
Now use a soldering iron to desolder the cables from the iPhone headphones, and undo the knot in the cables.
Step 5: Very Carefully Desolder the Sennheiser Headphones
Step 6: Unknot the Cable From the Sennheiser Body and Pull It Out
Now unknot the cable from the Sennheiser headphone body and pull out the cable and dispose of it. We will only need the plastic shielding and headphone body.
Step 7: Thread the Sennheiser Body and Plastic Shielding Through IPhone Cable
Now first straighten the iPhone cable and carefully thread the Sennheiser body and plastic shielding from the previous step through the iPhone cables. Make sure you have the left and right headphone cable marked so that you thread the correct plastic body through the correct cable.
Use some saliva as a lubricant and also to clean some of the glue on the cables etc. In the end fasten a knot at the end of the iPhone cables, preferably at the same place the original knot was.
Step 8: Solder the Sennheiser Headphone to IPhone Headphone Cable
Now the crucial step. Solder the Sennheiser headphone to the iPhone cables. Repeat for second headphone.
Step 9: Test If the Headphones Work
Now before closing everything up, its a good idea to test if the headphones work.
Step 10: Close Up the Headphones
Now pull the cables till the knot and close up the headphones. Finally push in the metal rings and you are ready to go.
Step 11: Enjoy Your New One of a Kind IPhone Headphones
So all should be working now. Enjoy!