Begin a conversation where you're making fun of her, in a such a way that you'll get her to make fun of you in return. Then when she does, (which is almost irresistible, once someone else starts), it's your opening to give an "Oh really?" and then give her a good fingering in the sides.
If you're lucky, she'll jump when that happens, and it'll seem natural to respond with a, "Look what we have here..." followed up with another 10-sec or so of tickling, before it might start to seem weird.
So while this isn't exactly a "session," I've found it's a good first step. I strongly believe the best way to get a girl into tickling is to make it seem like you're discovering it together, i.e. because it just happened one day, rather than that it's some deep, dark fetish that you've been harboring for years.
Good luck!