I am trying to cover a rice krispie shape with fondant, but it looks bumpy from the cereal. How can I get a nice smooth finish on the fondant?

You need to cover it with buttercream or your icing of choice first to get that nice smooth surface.

YOu can try covering with a buttercream first, use a crusting recipe and smooth it out with hot spatula, viva method...etc then cover with fondant. GoodlucK!

I'd like to know that also. I'm covering a silo (for a barn!) and have never done RK before.

I believe that most often the rice crispies are crushed finely then made into the treats, molded and covered in buttercream or royal icing then wrapped in fondant.

HTH

Is it fairly easy to work with RK? I really want to make a little train for a baby shower cake and I wanted to try to RK.
So you should crush up the RK's before you actualy add marshmallows and stuff??

I use a little less RK when making, so you have a little more MM. I keep manipulating the RKT to make as smooth as I can before covering w/ BC.