As many are seen evolving around, it's possibly a usefull tool to see which level of the class comes with which state of evolution.
Disclaimer : These are tips and pointers to aid you, if you wish to go totally by your own standards, you are free to do so.
Before the Class is even taken, a character must be made aware of it's arcane blood.
This can be done in various way, but the class isn't a passive one, like a priest not knowing he can cast prayers.
At level one, the dragon has been taken into Disciplehood by an already existant Half-Dragon or Full Dragon. Those without heritage, the true Dragon Disciples, are incapable of passing their gift onto others. With the first level, and thus the training, the taken disciple awakes next morning from their sleep, with a tingling sensation to their skin. On closer examine, they'll find they are already growing extremely tiny scales, with the colour of their own skin still. The tiny scales grow harder over the next days, and start taking colour after a week or two.
At level two, the now truly evolving disciple discovers he has a great lust for meat. Up to seven full meals *human standard* can easily be digested a day. Their appetite shifts to slightly rawer meat, and much more as well. These eating habbits are a requirement for their body's growth. Extremely powerfull surges of musclegrowth are starting to begin. Remember, a +2 to your natural strength is the difference between lifting a chair and a heavy couch on your own.
At this state, the throat of the disciple starts to irritate, they are often seen swallowing various oils, melted butter or loads of alcohol. In detail, their throat is irritated by the develloping gland which produces their breath weapon.
At level three, the breath gland becomes active, first, without control and pain after using, it's effects depending on the type of dragon of course. Further devellopment of the body of course doesn't stop. The disciple also starts to notice a sleeping pattern disorder, as they slowly start needing less sleep.
At level four, the disciples body again starts to require more food, as in more meat. It's again starting to devellop a strong surge, the blood slowly starts to warm, their body temperature rises about 5% above normal human body temperature. They feel extremely fit and require a lot of physical training due to their great amount of energy.
At level five, their scales start to grow slightly larger, and become more visible to anyone paying attention. Where they grow is dependant on the strength of the bloodline, and partly a player's preferences. Also, difference between warriors and mages can clearly be seen as warriors receive a thicker layer of scales on more places, since they'll be using them as true armor. Mages/Bards usually devellop a thin layer which is as strong as the other, however with less potential armor qualities. It's mostly better resistant against magic.
At level six, the disciple's sleeping pattern is greatly disrupted, yet he has a large load of energies left. His personality also starts to devellop some draconic habbits. Lazyness, Grumpiness, Personal Space and such are examples of the various habbits they slowly start to devellop.
At level seven, the dragon's body starts to grow harder and harder, the scales become more resistant and they start gaining their natural glow. At this point, they slowly start to get a feeling in the midst of their back, which slowly evolves to an actual pain.
At level eight, their body is almost completely evolved, yet the true strength of it is slumbering behind a veil that can only be passed by time.
At level nine, the pain in the back has become such a burden it will burst open, and wings sprout out. Covered in blood, with blood coming heavily from the large wound, this is extremely painfull and very messy. Not for the faint of heart. After this, the disciple finds himself/herself greatly relieved and notices his mind is now also free from the complex pain, and somehow capable of grasping deeper concepts of reality.
At level ten, the transformation is complete. The draconic heritage is fully awakened. The dragon can now live without sleep, his body is immune for his natural types*see dragon properties* and he/she walks with the grace of their ancient kin. Their full potential is awake, giving their body a final burst of strength.
Of course, there is a lot more to devellop, but these are as said, basic pointers.
EPIC Dragon Disciples are quite a different thing, as they surpass every border of normal dragon discipleship/halfdragonhood. I advice you talk to a DM before taking such levels, as goes for every epic prestige class, since it's not a simple matter