I am very new to cake decorating,learning everyday.I was told box mix will hold up to fondant,well it didnt work for me it crumbled even after chilling overnight.So please give me some tips.

I would try adding this recipe to the box mix. It definitely helps.

http://cakecentral.com/recipes/1977/cake-mix-extender

http://cakecentral.com/recipes/7445/the-original-wasc-cake-recipe

I just used this one for a cake with fondant and it was awesome! Received many rave reveiws on the taste also.

Thank you! How long should this chill?

I've never come across a recipe that didn't hold up fondant.... and I've used a ton of different ones.

Are you using and especially thick layer of fondant?

No its marshmallow fondant.I rolled it 1/4 inch thick.When I iced them with buttercream they crumbled

Oh, it crumbled while icing it, before the fondant?

I only use box mixes when doing a cake for a demo in a class, but when I do I find them annoying to ice. They tend to be crumby. But once iced, they shouldn't fall apart. Perhaps something went wrong with your baking of the mix?

How soon after baking are you icing them? How stiff is you buttercream? Try rolling your fondant about an 1/8" thick.

What brand of cake mix did you use? I have never had a problem using Betty Crocker. I teach and have had students have problems with Duncan Hines and Pillsbury. So you might need to dense those ones up a bit.

Also, did you crumb coat and is your icing thin enough? If it's too thick it makes it really difficult to frost. Or if your cake hasn't cooled completely.

Good luck!