It is best explained by video: http://youtu.be/NBtungm8-t8
1.) Install a desired version of Pocketmine.
2.) Download and install the PMRealms.php plugin.
3.) Edit the server properties to you desire settings. You must have a port forwarded to allow external players! You must enable query port (default is on)
4.) Run the server at least once. It is okay to see errors By this time. We run it to generate the PMRealms config file.
5.) Close the server and go into the plugins>PMRealms>config.yml
6.) Edit the file to your needs. The format should look like:
ownerName: (user name)
externalAddress: (Your public IP address or Host)
externalPort: (the port you assigned in the server properties file; default is 19132)
7.) Save the file and run the server, there should be NO ERRORS. The [NOTICE] is NOT an error but a friendly reminder.
8.) Play on your server, invite players to play, make a public server/network!