Here is a tip that I like to use- I like to pipe with a star tip.
So once you finish filling your cake. I like to pipe the whole cake using the star tip. Then smooth it out with your offset spatula and a flat edge bench scraper. This gives a even layer on your cake and easier to smooth out.
Plus, once you get your crumb coat on your cake (1st layer of icing) place it in the fridge to let the icing harden up. Then take your bench scraper and smooth out the icing (only apply light pressure) This will smooth out your icing to get the smooth edges. Then place your fondant on your cake. I hope this helps. Good Luck!! :)
Danielle - Sugar's Silk™
