What kind of cable would be suitable for wiring an MFJ-564B Iambic CW Paddle to an Icom IC-718?
I have heard that you can just use a length of old electric guitar cord but I was not sure what to do.
I always use stereo patch cables for connecting my paddles. The have the right number of conductors, three. Almost all stereo patch cables have two conductors and braided shield which is your third conductor.
This works nicely with the standard 1/4 inch stereo phone plug expected by the 718. You can go down to Radio Shack (before all the stores close) or your other favorite supplier of such things and buy yourself a 6 foot stereo cable with one end attached to stereo 1/4 inch plug.
Only issue it to make sure you wire the TRS (tip-ring-sleeve) to the right conductors and the IC-718 manual tells you how to do that. You should not need to rewire the actual 1/4 inch plug, just make sure that tip and ring are connected to the right spots on the paddle. The sleeve is almost always the ground braid attachment (but, double check with continuity tester anyway).
One minor little thing is that these patch cables use small and fragile wires so you need to be careful when hooking up. Probably solder on some nice lugs for hooking to screw attachments on the paddle.
What I like best of these cables is that they are usually very flexible and that is nice on a crowded desktop.
Assuming your question is literally what kind of cable and not what connectors do I need, the answer is: it doesn't matter. Use what is at hand, or what will lay nicely on your desk, or what looks good, or what's cheap at the store, as long as it has two (for a key) or three (for a paddle) conductors.
A CW key or paddle is just a switch or pair of switches, nothing more, acting as part of a simple DC circuit. This is the least demanding application for wire or cable that exists. (Exception: Some very old transmitters had high voltage on the keying circuit. In this case, the cable's insulation must be adequate.) You definitely don't need to worry about impedance.
Note that an electric guitar cable as you mentioned is not suitable for this case, because it has only two conductors and contacts, whereas a paddle requires three contacts (one common and two for the two switches). If you want to start with a prebuilt audio cable assembly, use a cable with a TRS (stereo) 1/4" plug. Or, use a 1/8" (a.k.a. 3.5 mm) plug together with a size adapter (these are readily available); this might be useful later if you use a different rig which uses the smaller plug.
Whether you use bulk cable or a preassembled cable, you will need to strip, separate, and solder the wires on the other end to the solder terminals on the MFJ paddle. (Some cables might have very fine wires that are hard to solder by hand or would break with use; if you build from scratch you can choose cable that doesn't have this problem.)