Solder the joints of the wires with a soldering gun.

For help with soldering, check out our How to Solder and Desolder Connections guide.

It may help to use a vice grip or similar to hold one end of the wires in place, so that you can manipulate the other side with your hands.

If necessary, you can rectify any blisters or spikes in the soldering using a small hammer.

Once you've soldered all of the connections, check with the ohmmeter that the current flows well from one end of the extension cord to the other. If it does not, check to make sure that the solder has made a firm connection with each side of each cable pair. It may help to reflow the soldered connections.