Test the downstream outlet by plugging in the GFCI tester and pressing the test button. Note: The lights on the tester should go out. Fold the wires back into the box and screw on the GFCI receptacle ...

Remember that there will typically be at least four wires to deal with—line, load, neutral, and ground—and that there will also be a traveler wire if you’re dealing with a multi-pole circuit.