You hold the icing bag with the wide end of the tip against the cake, and the thin end angling up and slightly outwards at the angle that you want the rufffle to go.
Try this on a cake tin before you start on the cake tier, to get the exact effect you want.
If it isn't already obvious, start at the top and work your way down.
There is another thread going right now about the same subject. This is what I posted earlier on that thread
There is a tutorial for this kind of effect on My Cake School . com Melissa Diamond uses buttercream to decorate her cake. This tutorial has the ruffle pointing up, http://www.mycakeschool.com/video-tutorials/buttercream-ruffles-in-shades-of-teal/
This one the ruffle is pointing down, http://www.mycakeschool.com/video-tutorials/tip-050/
If you want to get the tutorials, there is a yearly subscription fee of $30.00. Well worth it. There are sooooo many tutorials and she explains everything very well