Determine the type of connectors that you you need. The usual type for microphone cables is called an "XLR," though there are also "TRS" and "TS" (in both 1/4-inch and 1/8-inch varieties), and "RCA." Your cable can involve any combination of these.
Buy connectors. You can buy them online or at any electronics or music supply store.
Buy cable. TRS and XLR connectors are "balanced" lines: they require a three-wire cable with a ground line. RCA and TS connectors do not. You can buy audio cable in bulk online and at some retail outlets.
Cut a section of audio cable to the length you need. Strip away the rubber insulation from each end to reveal at least a 1/4-inch of wire.
Match up the bare wires to the appropriate terminals on the connectors. Be careful to match the terminals on both ends of the cable. Typically, the red wire connects to the hot, or positive, terminal, while the white wire connects to the cold, or negative, terminal. On three-wire balanced cables, the third green or uncolored wire, is the ground wire.
Wrap the wires around the corresponding terminals on the connectors. Solder the wires into place.
Assemble the jacket or casing around the connector ends of the cable to protect the wires and soldering.
Plug the finished audio cable into an amplifier, stereo jack or other audio appliance. Make sure there is power running to the appliance.
Connect the red and black terminals of the line tester to corresponding terminals on the audio cable connector. If the light turns on, there is power running through the cable.
Unplug the audio cable and store it for later use.