Instructions
Go to vbalink.info and download VBAlink.zip. This is a compressed file containing the VBA Link Add-on.
Extract the contents of VBAlink.zip to the folder on your PC that contains Visual Boy Advance, then open VBA.exe to launch Visual Boy Advance.
Run your "Pokemon Emerald" ROM in Visual Boy Advance and travel to the nearest Pokemon Center. If you are stuck in the wilderness, select a Pokemon with the "Fly" ability to get to a city quickly. Run "Pokemon Ruby" and do the same.
Enter the Pokemon Center in both games and speak with the man you find on the far right of the Pokemon Center. Respond "Yes" when he asks if you would like to link your "Pokemon" games and trade from "Emerald" to "Ruby."