The new RDDs are subraces in and to themselves. They require a pure race to start, no subrace in the subrace field. Get as much XP as you need to take your first RDD level. When you take that level, it will tell you must first STUDY a Dragon in the CHAMBER OF DRAGONS. It takes any extra XP you have, and sets you back to just enough to get your FIRST RDD level. But, in order to open a door inside that chamber, you will have to find your dragon's TOKEN. It is the KEY to getting inside. You must also learn the correct ALIGNMENT of your dragon.
Once all of these conditions are met you USE your STUDY tool on the dragon behind the door that you just opened with your TOKEN in the CHAMBER OF DRAGONS and then you start the process. At this time your token disappears, as does your STUDY tool. From now on, you can level when and as you see fit. Upon taking your last RDD level (10), you obtain your special appearance and full abilities of that appropriate type of RDD. If you wanted to go higher than level 10 in RDD, that's still an option, but your appearance and main abilities confer at level 10.
Each one's different, so enjoy. I hope this helps. If in the game you need additional informatino, seek out the book in a magic shop close to the city of Micidal. Oh, by the way, the CHAMBER OF DRAGONS is also close by to the city of Micidal.