I don't think deporting any of them makes any sense until there is first a well-guarded, physical barrier at the border to prevent them from coming back. We'd just be wasting time and money.
It is wholly unrealistic to think that we are going to find, let alone round up and deport 12 million individuals. But we damn sure can and should be deporting (after the border is sealed) any illegal who is caught in the comission of a felony or a repeat misdemeanor. That way, we're not wasting time tracking the illegals down. We wait until they come to us, so to speak.
The trick is to start deporting and securing the border at the same time, eventually we would have it down to a trickle coming in(you are always going to have some sneak in, somehow, it is a given)and a flood going out. No reason we can't do both at the same time. Secure and deport, start now!