⬢Place the black wire above the pin with "GND" sign. Make sure you are using the rounded connector.
⬢Place the washer above the rounded cable connector.
⬢Press the M3x10 screw through all parts.
⬢Hold the screw and carefully turn the heatbed upside down.
⬢Place the M3nN nut on the top of the M3 screw and tighten it slightly.
⬢Turn the heatbed back, using pliers and Allen key tighten the screw. We need to adjust cable position in the next step, therefore do not tighten the screw too firmly.
⬢Repeat this procedure for the second (red) wire.