1st: ice the cake and make random 1/2 swirls  with a tip of a spoon and rotate them as you go

2nd: ice the cake and use a fork and run the tips of prongs into icing to make squiggle/straight lines (squiggle is more forgiving)

3rd: ice cake and use tip of spoon and  dig little trenches going around the edge and random on top

4th: ice cake and use a butter knife tip and drag it from bottom to top around edge and from edge into the center on top

Now you never have to worry about making the icing all nice and smooth (which will give you a quick headache). Use these designs with butter cream or any soft icings like cool whip icing. I hope you enjoy!