It's really not as hard as it sounds. Get Tortoise SVN. After you've installed it, new options will appear in the right-click context menu. After you've opened your Beyond the Sword\Mods\ folder, right-click and choose "SVN Checkout". There you have to enter the link given in this thread. The SVN client will then download the mod folder. You only have to rename it to the usual name "RFC Classical World".
Afterwards you can treat it as a normal mod folder, the only difference is that you can always right-click it and choose "SVN Update" to get the latest changes.
Afterwards you can treat it as a normal mod folder, the only difference is that you can always right-click it and choose "SVN Update" to get the latest changes.