Summarize this article:

For this simple project, you’ll need:  Two tin or aluminum cans, or two papers cups Five-to-ten meters of string A hammer A nail Two tin or aluminum cans will hold up better than plastic ones. The string is less likely to tear through the bottom of a tin or aluminum can. Make the holes in the cans large enough to feed the string through the holes. Take the entire length of string and thread one end of it into one of the receivers. Make sure you feed the string from the outside of the receiver through the bottom and into the receiver. Each can will be act as a receiver. Pull the string through far enough that you can tie a good knot in it without having to reach inside the receiver.  If the first knot isn’t large enough to keep the string from falling out of the nail hole, then knot it again. If you’re using plastic cups instead of cans for your receiver, tie the string to the nail and leave the nail inside the cup. This will help keep the string inside the cup, as the string is more likely to tear a larger hole in the plastic. Make sure you secure the string in the first side before threading it into the other receiver since it will simply pull away from the first one. Now that you have finished your first receiver, you need to thread the other end of the string into the second receiver and secure it. Again, if you’re using paper cups for receivers, then you may want to get a second nail that you can use to help tie off the string. All sound is created by sound waves traveling across a substance. This is the same for voices and even sounds vibrating down stringed instruments, such as violins or guitars. So, you want the string to be stretched tight like a violin string in order for the sound to travel across it effectively. Of course, you don’t want to pull so tight that the string breaks or pulls the string out through the bottom of your receivers. Just pull the string tight enough that you can pluck the string and it will twang. Now that you’ve finished the setup for your walkie talkie, use it communicate. Speak into the receiver as the other person listens on the other receiver and send some secret messages.  Do not pull the string connecting the paper cups or aluminum/tin cans too tightly when using the walkie talkies. Too much force could cause the string to pop out of the cups or cans. If you make the receivers with tin or aluminum cans, be careful when placing your ear or mouth to the cans in case there are jagged edges around the lip of the cans.

Summary:
Gather your materials. Poke the nail through the bottom of each can/cup. Thread one end of the string into one of the cans. Tie the string inside the receiver. Repeat steps 3 and 4 on the second receiver. Stretch the string out until it is taut. Talk to the person on the other end.