Yours isn't the first time that question has been asked. I searched for the answer but couldn't come up with the answer.
My opinion is that it should go on with the edge you can see toward the rear. The upper slot would go on the front part of the headliner so you can tap it in place, then the rear part of the headliner goes in the lower slot. That way, the wind won't catch it and rip it out.
I'm sure someone will be along with a more solid answer. I'm interested in that I have the headliner install ahead of me as well.
Here's a pic: Link
[on edit] The Tech Tip
says the rear half goes into the upper slot (that's reverse of what I said.)