I've tried to cover several cakes with fondant but everytime i do the buttercream leaks out of the bottom in big globs. what am i doing wrong?
It sounds like the buttercream is being pushed down and out as you smooth your fondant. You could try waiting for it to crust (if it is a crusting recipe), then lightly spritz with water to make the fondant stick. If its not a crusting recipe, and you don't want to reduce the amount of BC under it, just scrape it off your board and discard. It won't show the mistakes if you do a border around the base of the cake.
you could try refrigerating your iced cake till the icing is hard, then cover it so you aren't moving the icing around as you smooth it.
You are likely using too much icing for your crumb coat (under your fondant - you only need a thin layer. And if you use Sharon Zambito's trick of putting it in the freezer for 10 mins after you do your crumb coat, you won't have that issue anymore. Works for me!