Identify the "Input" port. This is typically labeled "input," "in," or "full-range input." This is where you plug your cable from your audio source. For example, if you are connecting a CD player, the CD player output would be connected to the "Input" of your crossover.
Locate and set the crossover frequency. The crossover frequency is the point on the aural spectrum (20Hz-20kHz) where the crossover divides the audio signal sending the higher frequencies to one output on your crossover and the low frequencies to the other.
You can set the crossover frequency on most crossovers with a knob or switch. For professional crossovers found in sound systems and studios, the knob or switch will likely be on the front of the crossover. For sound systems such as car audio or home theater where the crossover frequency is not likely to change, the knob or switch will likely be on the back of the unit.
Identify the outputs on your crossover. The audio you want to go to the bass or subwoofer speakers will likely be labeled "Bass," "Low," or "Low Pass." The output for the high or full range speakers will likely be labeled "Treble," "High" or "High Pass."
Connect the input components. Connect the output of your audio source to the crossover with the necessary cabling. For example, the audio source will be a mixer in a night club setting, a DVD player in a home theater, or a CD player in a car audio system.
Professional sound systems will be connect using standard 1/4-inch cabling. Home theater and car audio systems connect with RCA cables in most cases.
Connect to your speakers or amplifiers. From your crossover outputs you identified earlier, connect the "Bass" output to your subwoofer(s) and your "Treble" output to your full-range speakers.
Power up the crossover. Most crossovers require a power source such as connecting to a wall outlet or the car battery. Plug in your crossover now that you have the audio source and amplifiers connected. Be sure your amplifiers are not on at this point.
Turn on your components. You can now turn on your other audio components, such as a CD player, mixer, DVD or CD player. The very last item you should turn on is your amplifiers to prevent loud signals being sent to your speakers when you turn on the various components of your sound system.
Test the audio. Play audio through your sound source. If the sound seems muddy, boomy, or you can hear too much bass; turn the "bass" or "low" channel down before you attempt to turn up the "treble" or "high" channel. You can adjust the channel with a knob or switch on the crossover. Professional audio equipment will have this on the front of the crossover while home theater and car audio systems will typically have this on the back.