This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
This section assumes that you have a basic understanding of how to
solder, so the actual assembly of the crossover is not discussed.
Rather, tips on choosing the proper types of capacitors and inductors
are given here.
To obtain low insertion losses, the inductors should have very low
resistance, perhaps as low as 0.1 to 0.2 ohms.
Also, be sure to select capacitors with proper voltage ratings. The
maximum voltage in the circuit will be less than the square root of the
product of the maximum power in the circuit and the nominal impedance
of the driver. For example, a 4 ohm woofer being given 100W peak will
see a maximum voltage of sqrt(100*4) = sqrt(400) = 20V. Make sure that
the capacitors are bipolar, too, since speaker signals are AC signals.
If you cannot find bipolar capacitors, you can use two polar capacitors
in parallel and in opposite polarity (+ to - and - to +). However,
there are some possible problems with this approach: the forward
voltage rating will probably not be equal to the reverse voltage
rating, and there could be a reverse capacitance as well. Both
problems could adversely affect your circuit if you decide to use
opposite polarity capacitors in parallel.