Use pruning shears to trim the plant, smoothing out the natural cone shape.
Step back and look at your arborvitae from all angles to decide how steep you want the slope of the spiral.
Tie a ribbon to the tip of the plant.
Wind the ribbon around the plant, marking the slope you desire for the spiral, until you get to the bottom.
Step back and look at the arborvitae from all angles, checking that the loops of the ribbon are even and represent the spiral you want.
Starting about half-way down the tree, use the hedge clippers to trim above the ribbon, moving towards the top of the plant. Remove the last 1/3 of each branch above the ribbon, and step back every few branches to look at your work. Stop when you are about 1 foot from the top of the tree.
Go back to where you started cutting and begin cutting the branches below the ribbon on the top half of the tree. Again, remove the last 1/3 of each branch above the ribbon.
Step back and look at the plant. The ribbon on the bottom half should match the pattern of the spiral on the top half of the tree. Move the ribbon if necessary.
Prune the bottom half of the tree using the same process you used for the top half.
Start at the top of the tree. Use your hand to follow each cut branch back to where it joins the trunk, and use the hedge clippers to cut branches you want to remove about ½-inch from the trunk. Check that you are cutting the larger branches you want to remove, not the smaller ones that make up your spiral.
Remove the ribbon.
Step back and look at the plant from all angles. Use the pruning shears to remove any uneven areas and more clearly define the spiral.
Use the pruning shears to trim the tip of the plant, blending it in with the top of the spiral.