On the Phone
Choose and treat a pro domme as you would any other professional specialist - doctor, lawyer, or mechanic. They're not your lover, they're someone providing a service using relatively rare skills. Some factors to consider:
Do they offer the specific techniques and style that interests you?
Do you have the right energy/chemistry together?
Pro dommes get lots of crank calls, time wasters, and guys hoping for free phone sex. You need to establish yourself quickly as a genuine likely session client. Be prepared to answer:
How did you find me?
What draws you to BDSM?
What are you looking for in session?
With whom have you had sessions before?
Why do you want a professional session?
These might not all be asked directly, but you should have thought about them beforehand.
Be entirely honest, e.g. "I've not really done much, but I've got these fantasies, and I think I'm ready to try them out." Then be prepared to tell the fantasy. This is not a time to by shy or reticent. If she's a good pro domme, she's heard it all before. More importantly, she won't be judgmental. She'll appreciate your efforts at articulating any forbidden fantasies. Likely she'll say "I can give you a session in that area". The worst she'll say would be "That's outside the services I offer right now, but I can refer you to someone who does that type of session."