Sounds like you are using separate components, and if you decide to use an external crossover, which I highly recommend, connection is quite simple. You connect your preamp outputs to the crossover inputs, then two of your crossover outputs will go to your sub(s) and the other two will go to the amp for the main speakers.
Some other crossover options are
https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd ($, digital)
https://dbxpro.com/en-US/products/223s ($$, analog)
https://dbxpro.com/en-US/products/driverack-pa2 ($$, digital)
https://www.xta.co.uk/wp-content/uploads/2025/02/DP648-Datasheet-Prelim-v1_1.pdf ($$$, digital)
There are other crossovers available, but I can’t remember what they are.
You can place your subs next to your mains if you desire. You will lose some room gain which may not even be an issue depending on what and how loud you play it, and it may help with potential phase issues.
Also, plug the ports on your B & W's
