Instructions
Read the statistics for any dragons you want to use and make sure you understand as much about them as possible.
Build up any dragon game, during a session or campaign, as you would any manipulative opponent. Make sure the characters and dragon have opposing goals.
Set up times when your characters will interact with the dragon in alternate forms. Try to limit these to talking or light combat, avoid heavy combat or anything that would alert the players to the creature's true identity.
Sprinkle the campaign with rumors of a dragon and a mastermind enemy; make sure to use a few other rumors to keep the connection from being too obvious.
Create encounters that lead the game towards one of the two enemies, keeping the fact that they are the same creature hidden.
Bring the two hunts together for a climactic end battle. Reveal the nature of the party's opponent and that they have met before.
Remember the dragon has had plenty of planning time for the fight and will likely have a wide range of devices and assistants to keep the party at bay.