Most kits come with a pre-designed crossover. You solder it, and it works. But designing your own crossover is the "final boss" of DIY audio. The book demystifies the filter slopes: 1st order (6dB/octave), 2nd order (12dB/octave), Linkwitz-Riley, and Butterworth alignments.
The biggest mistake at the intermediate level is assuming that a 2nd order Linkwitz-Riley filter is always the answer. A 201-level text will immediately throw you into the deep end with (Zobels) and L-pads . Speaker Building 201 Book