To go active you need a channel of power for each individual speaker, as well as either an individual , independent xover control for each speaker or each pair of drivers.
This results on the need to have a bandpass xover for your midbass (mid/high freq cutoff and mid/low freq cutoff)
You can do this many ways.
The cheapest solution is use a combo of your deck and amplifier's built in xovers.
Not the most ideal, but the most common way to try the active thing.
In a 2way component setup:
For starters you defeat the the HU's front chs xover (pass) and use the HP xover on the amplifier to drive your tweeters.(thats if amplifier allows up to 2k-5khz settings)
For the mids you have to cutoff the driver on both ends- use the HP on the deck (normally 63-200hz avg) and set the HP. If your amp alows you to have a high freq LP adjustment, cut off the LP on the high end. Adjust slopes by ear, just be familar with the safe usable freq range of the speakers.
Obviously you other choices involve spending money- a deck with active control , or add an external processor. these allow fro other tools to tune this active setup- EQ, level control, Time alignment...etc..