Measure the total width you want for your plant (or your plants if you want them all the same). Get a rope and size: half the width you measured before, plus 25 cm.
Put a sign on the rope, to separate your 25 cm and your half rope.
Bind the rope (using all 25 cm) to the central branch of the tree, in the central point. If necessary, check that the heights of the different plants closely dovetailed. You have to make a node wide, so that it can slide around the branch, as the collar around the dog's neck, but not up and down.
Make some attempts around the tree to verify that the node is at the center.
This is the only thing you have to do by eye.
Then you can easily pass the rope among branches, stretch the rope in every free space, around the shaft, and then cut the branches in the point of tension, ie at the end of the rope.
I think it would work. This is like to use a compass.
NOTE = A "lollipop" plant (Ligustrum vulgare)
These pruning "artistic" require a skeleton of iron.
Any evergreen plant with small leaves, which can be pruned easily, can become a "lollipop". Obviously "lollipop" is not the name of the plant but it is a "friendly name", referred to its form after pruning.
The Privet (Ligustrum) is one of the most widely used.