Check the output connectors for the master output section of your mixer. These are typically XLR, 1/4-inch or RCA. Acquire two Y cable splitters that each have one male end of the appropriate connector to two female ends of the same connector.
Check the input connectors for your sub amp. These typically are XLR, balanced 1/4-inch, unbalanced 1/4-inch, and occasionally RCA. Acquire two cables that connect from the output of your Y cable splitter to the inputs on your sub amp.
Unplug any cables that go to the main speaker amp or powered speakers from the master outputs of the mixer. Connect the Y cable splitters into the master output. Plug the cables to the main speaker amp or powered speakers into one output of each of the two Y cable splitters. Connect the cables to the sub amp into the other outputs of each of the Y cable splitters.
Bridge the sub amp per the instructions in its manual. This usually is as simple as flipping a switch on the back of the amp. This ensures that both channels of the stereo signal from the mixer's master output reach each subwoofer you attach to the amp.