connect the two black wires from the relay to your terminals on your ramps/rumba/etc that your heat bed used to be hooked up to + and - don't matter here. mount the relay board to the printer. Apply 12V + and - to the input terminals as marked on the relay board. use decent sized wire such as 10 ga speaker wire or lamp cord. connect your bed to the output terminals- lead polarity doesn't matter. on the Amp draw I would just test it out. If the power supply cuts out you're over the line.