I have my first paid wedding cake in a couple weeks. It is a 10 inch and 8 inch square stacked covered with fondant. I know how to do that. She wants the top tier, a 6 inch round, with pillars. I have the separator plates and pillars. How do I make the separator plate stay on the 8 inch covered with fondant? Just some buttercream? Any other ideas? Silly me I know you all have LOTS of ideas!

I would use pushed in pillars then you only have the plate under the 6 inch, and you won't have to deal with doweling the 8 inch

I agree with peg818, Push-in pillars are the best way to go!
And they are very simple to use.

Bless the person that created push in pillars!!! I love them.