Make sure you have good support. I would use the wooden dowel rods you can get from Michael’s, and make sure you use enough. Also, you might be using too much icing on the outside of the cake, the icing should be inside not outside. I use just enough so the fondant will stick to it, otherwise when the cake warms up to room temp and the icing softens up, the weight of the fondant will collapse it a little. Make sure the cake is cold and the icing hard when you cover with fondant.
I’m still working on getting sharp edges on my fondant-covered cakes and I’m getting better, so just practice.
Hope this helps =)
