Playing with the emulator online it's not so difficult but it it requires some time to dedicate. So, don't worry if the first time don't work properly. Ok, let's go!
1. First of all you need the right emulator, obviously. Get
here the latest version of the VBA Link (v1.8.0 beta0). VBALink is a normal Visualboy Advance, but it supports up to 4 GBAs linked together (2 when using Wireless adapter).
2. Now we will focus on the online connection. Get the program named Hamachi from
here. Hamachi is VPN (Virtual Private Network) client that allows you to create fake LAN connections making your computers on the Internet believe that are connected on LAN. See this page for further information about Hamachi and how to use it:
https://secure.logmein.com/products/hamachi/howitworks.asp
3. Tell your your friends to do the same you just did.
4. Each player open the VBA Link and setup all the things, like controls, save paths and so on. Important: make sure that the "Pause when inactive window" option under Options -> Emulator is unchecked.
5. Now each player go to Options -> Link -> Settings and Check "Network". If you want better speed at the cost of slightly less stability, select "Faster" on the slidebar (recommended for wireless Pokémon). Moreover you can also change the Link timeout but if you do so remember to apply the same value for all players.
6. One of you goes to the connection button on Hamachi (the one that looks like a triangle on the left) and create a new network, give it a simple name and a password.
7. The others go to the connection button and join existing network, they will have to put the same name and password that has just created, then click OK and they will be connected.
8. The same person who created the newtwork on Hamachi must select "Server" tab on VBA Link Options and click Start button.
The others choose "Client", enter the IP address of the joined network and click Connect.
9. When all players are connected, a message box with the text "All players connected" will appear (at least it should :-).
10. Load the game and play! NOTE: When someone closes the emulator, the others will become disconnected.
Some connection problems may occur, here are some possible solutions:
* Close any programs that's using your Internet band such as browser, p2p etc. except for Hamachi and VBA Link, of course;
* Either turn off your firewall or allow it to connect and accept incoming connections for Hamachi/VBA Link;
* Change the Safer/Faster range option and/or set a different port number.
Enjoy! ;)