Watch this video for a quick and easy demonstration: Demonstration
Go to the top right corner and click on edit.
Click on External.
Find an online server by searching "Minecraft PE Servers". Name your server.
Enter the IP address and port.
Now click Add Server.
Step 6: Troubleshooting
Sometimes the server doesn't connect right away, try again a couple of times.
Some servers require to type out: /register before you can play.