Ok....
Here's how....
Go to Disney Castle ,The cornerstone of Light(Even if you finish it)
Transform into Valor or any Form
Go to the Door (The door to the TIMELESS RIVER)
And Presto! You have a Genuine Full Drive!
There's heartless there so it's better to Level Up there
Or
You can Exit the world (Don't worry,the Full Drive won't go away)
+
To obtain the Final Form,all you have to do is...
Go to THE WORLD THAT NEVER WAS or TWILIGHT TOWN
In TWTNW,Go to the NOTHING CALL'S
U will start to Transform to a MASTER,Bet all Nobodies until your in the tippy top of the world.Do it over and over again!
Until you accidentally Transform into a FINAL FORM
For TT,Go to the Train Station (sorry i forgot the name)
When you arrive in the Yensid (if you READ IT BACKWARDS,It's DISNEY!)
Just in the entrance
Like TWTNW,U will just go back and forth

I hope my cheats can help you...