What exactly do you mean -- freshly whipped cream? Redi-wip? Cool whip? Something else entirely?
For anything that is in a bowl or bowl-like container, if you want to cover the surface, just blop it on with a big spoon, then spread it with an offset spatula. Put it in a pastry bag and squeeze out designs if that's what you need.
As for stuff in an aerosol can, like Redi-whip, that's a lot trickier. You've got to hold it pretty close to the surface you want it on, and then not jiggle it as you work.