1. Download and install: http://www.curse.com/addons/wow/fzxc (It goes in wow/interface/addons)
2. Add a bunch of people here to your bnet friend’s list. You probably don’t have to add everyone, but the more the better coverage you get.
3. After adding people to your friends list, right click on their name to set a note. Make that note #oldmenraiding
4. Launch WoW and just do /join oldmenraiding. The mod should be working at that point. If you want to confirm, you can do /fzxc info.
5. Now when you chat in that channel it’ll get sent to everyone in that channel across all realms.
6. If you have alts, all you have to do is make sure the addon is loaded and you join the channel.