Are you thinking about a friend, or a professional?
Honestly, in my experience, you're going to have a way easier time finding an amateur who knows what they're doing than a professional. Professionals, even those specializing in special effects and character makeup, tend to be pretty lost on this because it's not "conventional"; it's a "niche market". A professional who hasn't done a lot of work with trans women is probably going to end up making you look more like a drag queen than anything else.
If you ARE looking for a professional, ask if they've ever worked with trans women before. If the answer is no, your best bet is probably to move along.
If you're just asking a friend who's good with makeup, just be straightforward with it. "I'm cosplaying [CHARACTER] and I want to look so natural at it that I get stared at in the restroom even at a convention. I don't have the makeup [toolkit/skills/whatever else you're lacking] to pull it off as well as I'd like; can you help me out?"
If they don't know how to do it but are willing to help, have them look up tutorials on YouTube by trans women. Those are going to be the best bet out there.