Lots of good advice, mainly on crossover points which are important. One of the things I found huge was getting the phase right. Without it there is a jumbled mess no matter what you do.
Probably the easiest way to do it is by ear. Hopefully you have a 0-180 degree phase adjust on the sub
Play music with a good solid discernable thump thump from a kick drum is good.
Play with the phase knob on the sub for a bit until you can hear differences in timing. Then adjust the phase on the subwoofer until the mains and the sub 'kick' together. Off phase sounds blurry or softer. 'In phase' should focus the timing to solidify the kicks. Timing should be pretty easy to get close. I went around and around with various other methods and found if it sounds right it is right.
Cheers