Okay it seems like everytime I use fondant on a cake, I end up smooshing the butter cream off of the cake and out the bottom. What am I doing wrong?

either:
using too much bc, or
using too much pressure when smoothing the fondant

You can pop the cake into the freezer briefly to set up the bc then quickly apply the fondant. Frozen bc is less likely to smoosh.

Ditto to what leah said. I'd also add that it helps to work the fondant very quickly into place. As your BC comes back to room temp and is warmed by your hands, it softens up and you're more likely to smoosh it out.

Well, I have been putting it in the freezer while I roll out my fondant (and let's be honest, it takes me a good while to do because I can't make a circle haha), and I work pretty quickly. I even put it in the refrigerator for a few minutes to firm up the fondant (as I read a tip here to do so), but it still smooshes out when I let it sit over night. Maybe it is too much bc. How much is normal to put?

I use about half a normal coat.

Thanks Leah! Is that about 1/8 inch or so?