Personally I wouldn't go through with it, but in case you do decide to, here's a tip.
I'd personally recommend you tell her, because she'll expect more from you than you can give and it can get messy from there. Be honest. Tell her you're not experienced but you're willing to try and learn. If she decides not to go for it then that says more about her than it does about you.
However, if you do decide to go through with it (regardless of whether you tell her or not), research tips online and make sure you remember one thing: it's all about the friction, not the in-out motion. She won't get anything out of it unless you're right on her G-spot, so make sure you figure out where that is (it can be different for some women). Since she likes dominant guys you might want to be a bit forceful (but respectful) and overall confident. I've written 18+ content before so I've learned a thing or two about it, but I've no experience either (and don't really want any to be 100% honest).