You're going to need three programs; the Visual Boy Advance, the VBA Link (which in the download includes the VBA), and LogMeIn Hamachi.
First download VBALink/VBA here: (EDIT!: For unknown reasons the official VBALink site, vbalink.info, is down. Whenever I made this horrible discovery I have uploaded VBALink myself) My FileSend VBALink Upload You're going to want version 1.8 LAN/Wi-Fi. (NEW!: Which is the one that is in the FileSend upload.)
Then you'll need Hamachi from here: https://secure.logmein.com/products/.../download.aspx You'll want the unmanaged version. (I haven't tried the managed version but I'm sure it works the same just with more options to controls multiple computers through it's program. I'll give it a try ASAP for this tutorial's sake.)
Quick description of these programs to ease your worried mind; Hamachi is a program that acts as a virtual LAN port. Don't worry it's safe, I've used it for a long time and Hamachi reassures you with saying that not even THEY can view any personal info of yours.
As for Visual Boy Advanced it's a GBA emulator and VBALink is the program that lets you link 2-4 VBAs together.
Now that you've got them downloaded, open Hamachi and install it onto your computer. After it's all finished, Launch Hamachi and you'll be prompted to put in a username. Choose whatever you'd like and you'll see something like this...
Now that you've got your Hamachi "account" up and running unzip your VBALink V1.8. And you'll see something that looks like this...
You and a friend should now download yourselves an amazing game from here at CoolROM (or download just one for yourself to trade your pokemon between saves). Now that you've got your game, open up VisualBoyAdvance and go to Options - Emulator - uncheck 'Pause when inactive window'
(MAKE SURE EVERYONE THAT YOU'LL BE PLAYING WITH HAS THIS UNCHECKED!)
Now, moving on... Go to Options - Link - 'Settings...' and you'll see this window...
Choose Network and the person that will be hosting the server will click the Server tab while the other player(s) will go to the Client tab. In server you'll see a 'Start!' button...
Which you press and wait for your friend(s) to join. If you're not the host, from 'client' type in the host's Hamachi LAN number and click "Connect"...
After everyone's in it'll tell you everyone is connected, click okay and then hit okay out of the "Settings..." box. Finally to start your game go to File - Open and open the game you and you're friends are about to enjoy in multilayer mode!
For your first try I'd suggest Mario Kart - Super Circuit because it works perfectly with VBALink. Also remember that not all games will run smoothly on VBALink. This is not a problem with your emulator or your Rom, but with VBALink's compatibility with that game.
NOTE: If you are attempting to play Four Swords make sure you put the Link Timeout (in milliseconds) at 10000 or above OR 200 or below. Link Timeout can be found in Options - Link - Settings... - General tab. Also know that Four swords WILL run a bit laggy but it IS still very playable. (I've only tried it with two players so if anyone has played it with 4 let me know how it works)
NOTE: Some people have found that checking the 'Wireless Adapter' in Options - Link and/or unchecking 'Synchronize' in Options - Emulator, has improved performance. If your rom crashes and is officially supported by VBALink then try this minor adjustment for some, hopefully, big changes.
THIS JUST IN! Mar. 9, '10 - I have created a Hamachi Network for us CoolRom-ers to play some VBA together (kind of did it because I got tired of playing alone haha). The network ID is CoolROM and the network password is www.CoolROM.com REMEMBER!: Hamachi's Network search IS case sensitive. If it gets full I will start to create more and update you guys in this thread.
If you noticed I missed anything (or if I do after reviewing it twenty more times) let me know so I can update it ASAP.
BTW this is my first tutorial so if you feel that I didn't go into enough detail, or whatever, let me know so I can make sure everything is very newbie friendly.
Also, if you liked it... Hook a brotha up with some rep!