It'll take an investment, but it's worth the money...
You need -
Glueshrink (heatshrink lined with glue, that melts when you shrink it on with the heat gun) & heatshrink
Variable temperature heat gun
Cardas HPSC Sennheiser plugs
Wire of some sort, 24-26awg
Plug of your choice (Neutrik 1/4 or similar)
Sleeving (Furryletters on ebay sells techflex braided sleeve, or Wirecare.com)
Soldering iron + solder - the more you spend, the easier it will be to use consistently with reliable results
Glue gun + glue sticks
Pliers to stretch heatshrink (if you're trying to get it over something that it's slightly too small for)
Various scissors, wire cutters, a good quality Klein Tools wire stripper (I recommend the blue handled one with the 22-30awg stranded range)
Soldering flux to brush onto contacts like the Sennheiser plugs which are rhodium plated and don't like to be soldered
Teflon plumber's thread seal tape (to protect the sleeving from getting snagged on the end of your cable when you put the sleeving on, cover the bottom end of the cable with this)
Masking tape
"Helping Hands" alligator clip stand, radio shack sells these, to hold your plug/cable while you solder it on.
Multimeter to identify which wire is which at the very end
Cut the wire to length, and then braid it however you'd like. Use masking tape to secure wires together before braiding and to secure them at the very end. You can go with a round braid just look around for pictures and instructions. Or make two twisted pairs of equal twist rate, and twist those together. Secure at the top somewhere for 10-14" earcup wires (so the cable is 4 wire for most of it, then splits off into two two-wire pairs at the top. Sleeve the cable after covering all the ends with teflon tape so you don't snag the sleeving. Sleeve the cable, and then secure the sleeving with hot glue, leaving the ends of the cable (last 1", etc) unsleeved. At the y split cover it with some glue shrink, then you can use a metal tube for extra roundness or just cover it with some regular black shrink, this part is open to interpretation so long as you secure the split with some glue/shrink/glueshrink so the sleeving ends are protected. At the top of the cable, for each wire twisted pair, you can now solder on the Sennheiser plugs, brush their metal contacts with flux first, strip & tin the ends of your cable (dip the ends in flux, then tin them with solder). Try to solder the plugs by putting some solder on the iron, then endeavoring to only touch the iron to the rhodium-plated solder cup of the Sennheiser plug where the wires are each inserted. You want to face the open part of the plug (where the soldering takes place) inward, since they'll face in when they're inserted into the headphone. People have come up with all sorts of ways to solder these but there's a few ground rules - not too much heat or time spent heating, don't melt the plastic, and do NOT grip the plastic (on your clip stand - hold it by the cable's wires instead) of the Sennheiser plug while you're trying to solder it as the whole plug heats up during this process and the clip will deform the plastic of the plug. The only surefire way to do these is to solder accurately, use lots of flux, practice, and own a $650 soldering iron like me. I got em on my first try so I'm sure with these warnings you can do. Cover the plugs' open area with hot glue or epoxy, then cover with glueshrink to finish them, shrink it with the heat gun. You may have to stretch your shrink/glueshrink, 3/8" to 1/2" shrink fits but be sure it's 3:1 shrink ratio or it'll look like crap. Now to finish the cable. On the bottom end of the cable, there'll be 4 wires. Strip & tin them all, then put on the bottom cap of the Neutrik 1/4" plug (or the body of whatever plug, put on whichever parts of it can't go on when you've soldered the plug part on already). Use your multimeter now to identify each wire. Consider the red HPSC plug right and the black one left. You'll notice these plugs have a big fat pin and a little skinny pin. The big fat pins are negative and the skinny ones are positive. Now, identify which solder contact corresponds to which part of the end of the 1/4" plug. For the 1/4" plug, the part that goes into your amp - the very tip is called the "tip" and represents left, the middle contact (between the two black bands) is "ring" and represents right, and then the part of the plug below that is the "sleeve" and represents ground. If you use your multimeter, set to continuity mode (the mode where it beeps when you touch the probes together, and beeps when you touch it to each end of a wire or anything that's connected, like one end of your cable and the other end.... then you can figure out for each mini plug you use, which of the 3 solder places corresponds to Tip, Ring, and Sleeve (TRS). Just touch one electrode to the tip, then poke at the various solder places until you hear a beep, and that's where the left wire needs to be soldered to inside the plug...easy. Solder the two ground wires (for left & right channels) to the sleeve's solder contact; the right positive wire goes to the ring solder contact and the left positive wire goes to the tip solder contact. Now just test your cable and make sure that the tip of the plug goes to the small pin on the black plug, the ring of the plug goes to the small pin on the red plug, and the sleeve of the plug goes to both the red plug's big pin and the black plug's big pin. Finally make sure that there is no beep when you connect tip & sleeve, tip & ring, or ring & sleeve - this way you know there's no short. You don't want those to be connected...now cover the soldered area with some hot glue and tape it up with teflon tape, reassemble the 1/4" plug and screw it all together, and you're done...
I'll type all this out once a year, as a holiday gift...
