1. L+R speaker-level outputs from amp to sub's L+R speaker+level inputs.
2. L+R speaker-level outputs from sub to L and R speakers.
3. Use the "crossover" control (actually a low-pass filter (or LPF)) on the sub to blend the sub with the speakers.
See also the owner's manual for the sub, which is available on the PSW10 product page.
Kudos to you for your patience but how - why? - have you managed to be so patient? A six-month wait might be "sh*t happens", but two years is nothing short of "f*ck you"!
Is Seaton even aware that they still owe you a sub? Maybe they lost your order or you otherwise, somehow, fell off their radar?
It's a decent, but very modest option. And it can often be had for under $100 when on sale. (I believe there was a sale at Amazon last year when that sub was selling for ~$90, shipped.)
If your budget can afford it, IMO you should consider the very well-rated Dayton SUB-1200 (~$150, shipped) from parts-express.com.