Many preamps have subwoofer outputs, and that would be the easiest and best way to hookup. Just plug in the subwoofer amp's input to the preamp's subwoofer output and set the crossover frequency. Keep it to 100Hz or under if you can, depending on whether your main speakers will go down to 100Hz.

If your receiever's preamp doesn't have a subwoofer output, then you'll need a summing amp and a low-pass filter. The left and right channels are combined in the summing amp and the low-pass filter forms the subwoofer crossover, sending only low frequencies into the subwoofer amp.

Actually, the "summing amp" doesn't need to be an amp at all. Just a mechanism to combine the two (left and right) signals to present to the subwoofer amp while keeping them isolated (not shorted together) at the preamp's output. This can be done with a couple of resistors - about 5k or 10k would be fine - having one end of each of the resistors tied together for the common subwoofer output and the other end conected to each of the inputs, right and left. This can then be connected to a summing amplifier, but it can also be connected directly to your subwoofer amplifier in most cases.