I need to do an apron on top of a cake with a ruffle...

basically coverin the top of the cake with fondant and just letting the excess hang over top to look like a table cloth i guess...

How do you accomplish this...? Do i have to cut it before putting in on the cake or is there an easy way to cut it once it's on the cake...?

I would cut it and decorate it before moving to the cake. Move it on a cake round or something else thin that has been dusted with cornflour, it will be easier to lift it up onto the cake that just using your hands. Slide it onto the cake and when you are happy with the positioning, stick it down with water/vodka/gum glue.