I am planning a birthday cake in which I will apply fondant cutouts. The cake will be iced with buttercream. My question is what do I need to use as glue in order for the fondant to stick to the buttercream?

You can use BC or royal.

I use buttercream only because I don't usually have a batch of royal icing lying around so it's easier for me, but I know both work great.
Good Luck!

It usually will just stick right to the buttercream (like dots, stripes that sort of thing)

Thank you all very much! You've been very helpful!

Regular buttercream, royal icing, or just water will all work fine. I usually just put a dab of buttercream and have never had a problem with anything falling off.

How do you make modeling clay out of chocolate melting candy (wilton type for moulding) I saw on somebody sight how to make & use it for Roses. I'm not good with fondant. My Daughter uses it. I help her.

I use vanilla or water down fondant and paint on and stick just like glue.