I've encountered this with a couple girls. Honestly I personally would not mind if you asked for me to teach you but then again, I enjoy French kissing a lot so I would want you to know how to do it.

Now that I'm trying to explain it I'm realizing it's pretty difficult for me to explain the technique in words but I'll try. For me, all kisses start as a peck. After a couple of these you can transition to the French kiss. The most important part here is that one or both of you has to turn your head to the side in order for it to really work. It's hard to French kiss straight on (as in both noses touching side by side). So once the heads are sort of cocked, open your mounting and literally stick out your tongue. Basically you're going to feel around for their tongue, and as they touch you sort of swirl them together. I will state that for me, this action tends to last about 1-4 seconds. It's a quick thing. You don't sit there with your mouth open for 30 seconds slobbering tongues, doesn't work like that lol. So it's basically like a succession of open mouth-stick out tongue-swirl for 1-4 seconds-retract tongue-close mouth, repeat. Break up the rhythm with some more peck kisses or neck kissing, then back to French. If you're getting lots of spit all over your chin, you're doing it wrong. Use the time in between kisses to control that, unless your SO is into sloppy kisses.

This is my attempt to explain! If somebody else wants to attempt that would be great. Hopefully I helped somewhat.