I know this may be out there somewhere but I cant for the life of me figure out how everyone gets a perfect round cake with no bumps or sag. It seems no matter what I do nothing works. Is it my cake recipe? I admit I have used boxed cakes and boxed pound cakes and when I buttercream them before fondant they look fine. But when I put on the fondant it seems to lose shape. not have crisp corners (round and square) and show the lines in between the layers. I hope someone can direct me to the right way. It seems when I watch the cake shows their cakes seem so dense and very easy to stack and sculpt but cant seem to do the same. Thanks so much in advance.

When I started I had similar problems with my fondant cakes. What fixed it for me wasn't so much the cake recipe, but making sure I had it nice and chilled after I crumb coated the cake. This helped hold the crisp edges while I was doing the fondant and kept it from getting squishy under the fondant and creating lumps . Also, I was making too much of a dam, I reduced that and the filling amount, and rolled my fondant a little thicker and haven't had the problem since.
Another tip I found that works for the edges is using 2 fondant smoothers.
Hope this helps...

I've found that the kind of icing you use makes all the difference. When I was just starting out and using shortening based buttercream, I had the same problem.

I switched to using only Italian Meringue buttercream or chocolate ganache. Both harden up nicely in the fridge and hold thier shape until the fondant sets.

One other thing that helps a ton is the way you ice the cake. I take two cardboard cake circles and put one on the bottom of the tier and one on the top (like a sandwich). I line up both circles to make sure they're even and then fill the icing to meet up with the edge of the cardboard. I then take a putty knife (usually a 6 inch) and run it along the outside of the cake. This makes the icing perfectly smooth and the same depth of icing around the entire cake. I put it in the fridge to harden, then I take a sharp knife just under the top circle to loosen it, pop it off, then cover the top of the cake in icing and put it back in the fridge. VOILA! perfect every time.

great tips. I have been using a shortening/margerine buttercream so maybe this will help.