It's real simple. so with most map makers we make the folders so it's easy to move them over or use JSGME.
Most maps will have the Media folder inside if not it will have Levels atleast.
Basically you move everything from Media - levels to media.rar - levels and if there are any other folders you move what is inside of them to the correct locations in the media.rar file.
Spintires.nl I don't think there are any maps out yet.
Not at the Moment but yeah the only side I would download from.
Reason: This side is directly operated by the modders, so there will be no scam, maleware or something else. (Hazzard)