As some others will probably suggest, try practicing on your own. I did that for years before having a Dom do it to me. The only experience I had prior to that was vanilla, having a couple of kinky girlfriends stick their finger in my ass while blowing me. . That's always fun. The thing with knowing what to expect with a Dom in a strap on session is it's not an across the board good or bad experience. I've had doms that are really intuitive and now how to fuck where our chemistry is good strap on wise (usually otherwise too) and others that weren't. A couple that had no idea what they were doing and even hurt me by pushing too deep, too sudden, too hard. That sucked. It's nice to find a Dom that in genuinely into fucking ass, someone who really loves it. A lot of them fuck ass for the money and aren't really crazy about doing it otherwise. I get it, it can get dirty and smelly if the slave doesn't come prepared and cleaned out etc. That can gross out Dom. Even if they have one bad experience it can tarnish their future S.O. sessions once that seed's been planted. Good luck to you, I hope you find what you're looking for and have a blast doing so.