Trim your arborvitae with an electric hedge trimmer to smooth and perfect its cone shape. It should be large on the bottom and come to a point at the top.
Attach twine or ribbon to the top of your arborvitae and wrap the twine or ribbon down and around the shrub, making sure that the laps of twine or ribbon are equally spaced and the spiral is at an angle you like.
Use your pruning shears to cut into the arborvitae, using the ribbon as a guide by pruning along the line of the ribbon. Cut to the center of the arborvitae, but make sure you are not cutting branches that are part of the spiral that remains.
Carefully round off the edges of the spiral after removing the twine or ribbon, then prune and touch up the shape in early spring each year.