how do you put fondant on a cake to get nice, sharp edges? mine are always kind of rounded.

There is an awesome video on you tube the lady shows. It's all about getting the buttercream on right she does a few thin layers of icing and freeze between each one. I always did one big thick layer and got rounded edges. I have also seen people put walls up of fondant on each side and top and royal icing or buttercream along the seam. Sorry don't know her name search how to smooth buttercream cake. I think that is what I searched.

The sharp edges are most commonly achieved by using ganache under fondant instead of buttercream. Hth

Also the upside down method of icing gives you crisp edges and smooth straight sides before you start covering with fondant. You have to make sure your undercoat is perfect otherwise the fondant will highlight the flaws.

See this video from "Inspired by Michelle"