Two-way light switches are very common in homes. It allows you the ability to turn on a light from two locations. This is especially helpful when you are going up or down stairs or going into a dark room. What you are basically doing is controlling the flow of electricity from the main power to the light. Since this article describes how to wire a two-way light, this will show you the steps with an existing power source and ground already in the box. It will also show with a light fixture already in the ceiling.
Test the existing wires in the box to make sure the power is off.
Strip both of the white wire ends. Attach the white wire to the white wire going to the light. Twist together and put a wire nut on it and tighten. You can also put a small piece of electrical tape around the wire and nut.
Strip the end of the power source black wire. Use the needle-nose pliers to bend the stripped wire in an arch. Attach the black wire from the power source to one of the screws and tighten. Do the same for the other black wire that is going to the light, and attach to the other screw.
Take the green wires (ground wire, sometimes they are copper) and strip the ends and twist together. Put a green wire cap on the end.
At the light, connect the black wires and put a wire cap on. Connect the white wires and put a cap on and finally put the ground wires together and put a cap on.
Put switch back into box and put cover on. Put light fixture up. Turn on power.
Always make sure the power is off before starting.