Wiring for this project is really simple. Just connect the RX of one board to the TX of the other, and the TX of one board to the RX of the other. Then connect the GND pin from one board to the GND pin on the other.
Make sure that it's wired RX-TX and TX-RX and not RX-RX and TX-TX.
Mixing up RX and TX is pretty common, since they are 'relative' designators. If your setup isn't working, try swapping them!
When you've wired them up, they should look something like this:
