Find the manufacturer's sticker on the back of the speaker. You will see a listing that shows the ohm load that the speaker supports. Most speakers will be between 2 and 8 ohms, but they can go as high as 30 or more. If there is no sticker, look up the make and model on the manufacturer's website to find this information. The ohm level is also often listed directly on the amplifier where you plug in the speaker cables.
Set the ohm level you want by choosing a speaker cable that supports a lower load than the ohm speaker supports. If you want a 2-ohm speaker on a 4-ohm system, just use a parallel cord to split the load between two cables. This way, each cable will only output 2 ohms and not exceed the maximum ohms of the system. As long as the number on the cord is half the value the speaker supports, you won't have problems. If you are using a single speaker cable, the cable must match the ohm level of the speaker exactly.
Plug the speaker cable into the amplifier system. Turn on the power and test your speakers at a very low volume. If you test the speakers at a high volume and you have used the wrong cables, you may find yourself with a set of blown speakers. If the audio quality is clear and not cracking at a low volume, you can turn the volume up and enjoy your speakers. If there is a problem, double-check your ohm level capacity on the speakers and cables, since there is likely a mistake.