Wires from your main speaker would be connected to the High level INPUT on the sub.

You may find that the level might not be high enough, if so then connect to the low level input.

The type of subwoofer was built before receivers had dedicated subwoofer outputs. The idea is that the output of the source would go through the sub then on to the speakers. Typically the signal to the speaker would go through a low pass filter so the main speaker would not try to reproduce the low frequencies that the subwoofer produces.