Set the crossover first. If it gives your any sort of reference, shoot for something like 80-125 Hz. This controls how high the sub will play (e.g. if you set it at 125 Hz, it won't play above that setting).
For the gain, start with the gain at 0, play a song, and turn your radio volume as high as you think you'll ever listen to it (maybe around 25?). Slowly turn the gain up until you hear distortion out of the sub, and then back it off a little to get rid of the distortion. If that's too loud for you, you can set it anywhere below that point.
MODS: PW TMIC, Cobb catted DP, HKS cat-back, AVO filter, Bren e-tune; Konis/Epics, Advan RCII