If you have a high end head unit, it may come with 2 to 3 pairs of RCA outputs. Front/Rear/Sub. So just direct them to the crossover RCAs.
In the other case, you have a preamp with a builtin crossover. in that case its a lot more complicated. you'd have to direct the signals to the preamp and then to the amp, then to the specific speaker.
Chances are your component speakers have passive crossovers. You can have one wire going from your amp to the crossover, and your tweeter/midrange/woofers will be split at the crossover.