How does a crossover work? How does it do that thing it does? It's fairly simple really, a passive crossover uses what's known as a filter to prevent certain frequencies from reaching your speakers. Filters are capacitors and chokes. The value of the cap or choke will determine the frequency at which the filter rolls off and those values are as follows: Farads for capacitors (usually given as microfarads) and Henries for chokes (chokes are usually millihenries) named after the people that came up with the concept of measuring the values. A choke is a coil of wire and will filter high frequencies by rolling them off at 6db per octave while a capacitor will filter low frequencies, also rolling them off at 6db per octave.
To make filters that roll off at steeper slopes you simple combine elements. For example, a 12db per octave high pass filter will have a choke that is connected in parallel to the speaker and a capacitor that is connected in series to the speaker. This will shunt the low frequencies back to the amp at 6db per octave via the choke and block the low frequencies also at 6db per octave via the cap. A low pass filter will be comprised of the same elements only hooked up in the reverse order. The choke will be in series to the speaker while the cap will be in parallel.
For even steeper slopes you simply add more elements to the crossover but bear in mind that as you add in elements you'll be changing the values for like elements because multiple similar elements will have an effect on the overall value. For example, two caps in series results in half the capacitance while two caps in parallel will double the capacitance. Two chokes in series will double the value while two in parallel will halve the value. This is, of course, assuming that identical values are used for the chokes or caps.
For designing crossovers I've found that WinISD Beta does a great job for giving you the design you're after along with all the values needed for the core components. You can also use it for designing notch filters which are great for taming any peaks you might encounter.
Have fun and if you have any specific questions feel free to shoot me a PM and I'll do my best to help.