Well, basically its the same as Garena.
Download and apply the latest patch (1.23a) on WCIII ROC, FT or both.
Go to eurobattle.net website.
Register and activate your ID.
Download the "Server Installer". It's links are given on the website.
Install this Sever Installer inside your WC III directory.
Run euroloader and then run the game.
Click on the magnifying glass kind of icon near Battle.net. Normally that shows the list of four official servers. Notice that there is a fifth server named "Eurobattle.net" on that list. Click on that and then OK.
Now Click Battle.net, it will connect you to eurobattle. Login with you username id and you're good to go.