For all done, we usually do the ASL sign for finish/finished because it is a little simpler. You start with palms facing in, then turn the hands so that they are facing out.
By keeping your baby signs simple, you reduce frustration and increase the speed at which your baby can successfully imitate baby signs.
All done (or finished) is a great introductory sign. You can use it at the end of every meal, and it allows your baby to signal when they are done with their food, instead of getting fussy sitting in front of their plate. "Are you all done?"
It is easiest to teach the all done sign in the context of meals. Once your baby understands how to sign all done for food, you can start expanding use to other contexts, such as when they are finished playing with a toy or listening to music. You can also introduce the flash card at that point to enrich their understanding.