Summarize:

For this battery, you will need one unopened can of soda (any type will do), one plastic cup (6 to 8 ounces), and one 3/4-inch-wide strip of copper that's slightly longer than the height of the cup. In addition, you'll need a pair of scissors, a voltage meter, and two electrical lead wires with alligator clips at both ends. If you don't already have these materials around your house, you can purchase them at a hardware store. You can substitute the copper strip with several pieces of copper wire stuck together or bent in a zig-zag fashion to reach the desired width. Note that the cup doesn't absolutely have to be plastic. It just has to be non-metallic. Styrofoam and paper cups will also work. Discard (or drink) any soda that remains in the can. Turn it upside down in the sink and give it a few extra shakes to make sure that all the soda is out. Cut a 3/4-inch-wide strip from the side of the soda can. Ensure that's it's slightly longer than the plastic cup's height; if this isn't possible, don't worry you can just bend the top of the strip and let it hang over the edge of the cup and into the fluid. Instead of cutting the can, you may purchase aluminum strips from a hardware store. Aluminum foil is not an effective replacement for an aluminum strip; don't use it! You can skip this step if you bought the aluminum from a hardware store. If you cut the strip from a soda can, you'll need to sand off the coatings (i.e. paint, plastic) on both sides of the strip. Ensure that the strips do not touch each other. Place them across from each other not side by side or overlapping in the cup. Ideally you've cut the strips long enough that their tops sit above the soda, extending slightly past the rim of the cup. If the strips do not extend past the cup's rim, you can bend each strip lightly so that it hangs off the edge of the cup. Attach one lead wire to one metal strip by opening the alligator clip and closing it on the strip. Then, attach a different lead wire to the other metal strip, again using the alligator clip. Be careful not to let the alligator clips touch the soda. It doesn't matter which color of wire attaches to which strip. Following the instructions that come with your voltage meter, connect a lead wire from each metal strip to the voltage meter. The meter should read your battery's voltage at roughly 3/4 of a volt.
Gather your materials. Fill the plastic cup roughly 3/4 full with soda. Make sure the soda can is completely empty. Cut a strip of aluminum from the soda can. Sand the aluminum strip (optional). Place the strips into the solution. Attach lead wires to metal strips. Test the battery.