Everytime I try to cover them with fondant it gets lumpy and shows through the fondant. I tried putting two layers of fondant but it still showed a little bit and I was wasting too much.

How do I make it look smooth with just one layer? or i have to use more?

You have to fill in the extra space. I ganache my cakes, so I ganache the rice krispies let it set, then cover it with fondant.   If you use buttercream under your fondant, ice the rice krispies in buttercream then cover in fondant. Hth

I agree with shyler Put a layer of bc or ganache under your fondant.

The smoother you can make the sculpture, the smoother your final product will be.

I use a tool similar to the one on the far left in this set (but mine is made of wood) to smooth the RCT before I cover it with any product. I press it against the surface and pull it along so as to slightly crush the surface.

http://www.michaels.com/Sculpey%C2%AE-Clay-Tool-Starter-Set/gc1584,default,pd.html

Before I got that tool, I used to use the back of a spoon to press the surface of the RKT as smooth as possible. The problem with the spoon is that too much pressure will leave a depression in the surface.

After I have it as smooth as I can get it, I cover the sculpture with a very thin layer of candy clay to fill any dips that might be left. I do this because it is not nearly as messy as working with buttercream which never gets hard, or with ganache or melted chocolate or candy melts which will melt from the temperature of my hand and wind up all over the place. I find that the candy clay can be smoothed more easily and with less mess.

The teddy bear was done with buttercream:

http://www.lamaisondesgateaux.blogspot.ca/2010/12/un-gateau-de-chocolat-couvert-en.html

The kayak etc. was done with candy melts:

http://www.lamaisondesgateaux.blogspot.ca/2011/05/bonne-fete-dan.html

Thomas was done with candy clay:

http://grandmasugarskitchen.blogspot.ca/2012/02/happy-birthday-yvonne.html

The easter eggs were also done with candy clay, then covered with gumpaste. They were so smooth that people thought they were real duck eggs:

http://grandmasugarskitchen.blogspot.ca/2012/04/basket-of-eggs-for-easter.html

HTH

I just did a rkt fish this weekend and I sculpted the basic fish shape out of rkt then covered with melted white chocolate. Then I put it in the fridge for about twenty minutes to harden...I then used a microplane to shave and smooth out the surface and then went over it with the heat of my fingers.