There's a guide on the nanoloop site:
I can't give a definitive guide since all of the cables I have are different in terms of the colour of internal wires used, but if you have a multimeter it's not difficult to figure out the pin/colour combinations yourself.
In terms of how to seal it back up again, heat shrink tubing over the soldered cables should do the trick. For the cables I've made, I usually put this in the middle of the cable; it seems to minimise stress that may cause the solder to weaken. Ideally, you'll want to put tubing over each soldered wire pair as well as over the whole thing at the end, to hold everything together nicely.
Another (better) option is to buy a phono plug by itself, like the following:
or a higher quality option:
The only disadvantage of doing it this way is that molded plugs (like in your link above) are generally more sturdy, but buying the plug by itself means that you won't need to use any heat shrink tubing—you can just have the original gameboy cable terminate at the plug, therefore reducing the amount of weak points. Also, it may be fiddly to separate the wiring in those pre-made cables; it tends to be quite delicate. Gameboy wires, on the other hand, are pretty oldskool in that you can just strip the insulation off and solder to something else pretty easily.
I hope this makes sense! I'm not too good at explaining these things.
Last edited by pselodux (January 6, 2016 11:28 pm)