Ok to start off this is in no way intended to be a replacement for buying a new chord or just falt out a longer one, it is intended for those who find themselve strapped for cash and have a broken intrument cable. Also read this entire document before atempting to fix a cable.
Ok to start I will describe to you the basics of the guitar cable. There are two groupings of wires iside these badboys, one lies in the middle of the cable and is shielded usually by a thin black layer of shielding and a thicker, more durable, see through blue/white shielding. The second one is wraped around the inner one and is only shielded by the shielding you will see on the outside of the cable. The two wires connect to the plug like so
1 = The conection point where the thinner inner bundle of wires connects to plug. (Referred to as the top conection point later)
2 = The connection point where the thicker outer bundle of wire connects to plug. (Referred to as the bottom connection point later)
The area where most guitar cables go bad is where they actually plug into the instrument/amp, I personally have never seen one where it was broken elsewhere, so this would be a good place to start. First if you ends have a shielding over them either cut them away or remove it by sliding it down the cable and if there are rubber rings around the cable under that resmbling a headphone cable cut it off. I personally have never gotten one to budge. Second you'll want to unscrew the actual metal protector, which may also have a little plastic tube under it which is easily sliden off. If the cable is broken here it should be at the top connection point altough it is possible that the bottom connection point could break but I'm not sure how that would happen unless the cable was simply ripped from the plug altogether. There are two ways of fixing this.
Steps 1-10 are best suited for if you have no other cables lying around with a good plug. Steps 11-18 are best suted for if you do have other cables around with one or two good plugs.
01. Take razor blade/ exacto blade (a scissors will work but will make this job much more dificult as you don't want to simply cut it or a wire stripper, although I personally am not in the possesion of one that will strip such a large or small wire and make a cut like so (make it about a half inch to an inch away from the end of the shielding):
On the cable, make sure to bend the cable in half to make it easier to tell when you have cut through, also if you feel the blade cutting through something that resists to a higher degree than the shielding stop imediatly
the wires inside are rather fragile and proceding to cut will produce quickly increasing amounts of damage to the bundle of wires. When your cut has made it through the shielding bend it in half so that about half of the bent area will spread the cut wider and icrease it's size. If you cannot do this you can simply proceed by cutting it once again. (you can also completely cut the cable and skip step 02, which makes it easier to have the two bundles be even in the end)
02. Once you have done this you will need to make another cut like so:
_ _ _ _ _|
careful with this cut as I have not found a way to make this easier and the cable has a tendancy to not want to stay put.
After this you should see something like this:
______|_____ _______ (Shielded wire)
03. The wire you just removed the shield from should either be wrapped around the inner bundle or will go straight through the cable, at thise point you should seperate the two bundles and twist the outer bundle of wires to keep them together and prevent problems with the two bundles making contact. (Which disables the cable from transmiting any sound at all)
04. Repeat step one, being extremely carefull as this budnle is much thinner and in some cases just extremely small and make sure the cut you make is not in the same place as the first one, make it slightly closer to the plug. This will prevent issues of the wire bundles from coming into contact.
05. Slide the shielding off of the wire bundle.
06. You may notice that the other bundle is significantly longer than the inner one, this problem can be fixed by wraping the outer wire bundle around the bottom connection point until the inner bundle reaches the top connection point and shielding the wires with electrical tape. (Duct tape and masking tape work fine too, I'm pretty sure scotch would work equally as well but I have not tried it.)
07. Reconect the inner bundle to the top connection point by either soldering it on or tapeing it back on and if you decided just to cut the cable in half the out bundle to the bottom connection point.
08. Make sure at both of the budnles are properly shielded. (i.e. wrap electircal tape or the above mentioned substitues, around them up to the point where they come into conact with the connection points.
09. Put the plug protectors back on.
10. Rock out.
Now If you want to make your cable longer, you will need another cable, does not matter if the size is not the same.
11. Cut cable roughly in half and cut the other cable at both ends. This can also be used as an alternative to the other way to repair damaged plugs, just cut the cable of at the damaged plug and the other one at it's damaged plug, or just anywhere if both are working and proceed as normal)
12. Repeat steps 01 and 05 on both cables making and both bundles sure to have the shielding of the inner bundle stick out from the shielding of the outer bundle and making sure to twist them until the shielding of each touches and twizting the budnles to keep them tidy.
13. Wrap the inner bundle of one cable around the inner bundle of the other.
14. Wrap the excess wire around the shielding.
15. Wrap electrical tape, or the above substitutes, around all exposed wire.
16. Twist the outer bundle until it is an equal length, subtactracting the excess of course, with the inner bundle.
17. Repeat 14 and 15.
18. Rock out.