Websites such as those provided by Crutchfield and Sonix Electroics have a number of wiring diagrams for those wishing to install subwoofers into their sound systems. Because of the variables involved, you should have information about the subwoofers, your amplifier and other speakers in the system. One you know how many channels your amplifier powers, the impedance and power of the amplifier, speakers and subwoofer, you can choose the proper wiring diagram to use install one or more subwoofers.
Wiring diagrams for stereo systems utilizing an amplifier and two speakers aim to match impedance and power characteristics of all the components. For two speakers systems, the choices boil down to two: series and parallel. Which you use depends upon your particular components. To get the best sound from you system when adding additional components, the number of options increases, as does the number of variables to consider when wiring it up.
There are also many other things to consider when using more components to avoid potential damage from impedance and power mismatches. When comparing power ratings, make sure to compare apples to apples. Some power specifications for amplifiers are expressed in root-mean-square watts, which is very different than the maximum power output ratings of speakers. Combining these without making the required conversions can result in damage to the speakers or the amplifier itself.