Pruning mature cedar trees should be done seldom if at all. The problem with trimming a cedar tree is that every cedar has a dead zone in the center of the canopy. The new green growth is dense. It blocks the sunlight from the older growth beneath and without light, it dies. The outer green growth does not extend very deep into the tree. If you are pruning cedar trees and you cut branches back into the dead zone, they will not regrow.
If you do decide to prune proceed branch by branch. Snip off the green branch tips on the first branch, making each cut above a lateral bud. Then proceed to the next branch and do the same. The key is not to go pruning cedar trees into the dead zone. Check before each snip to be sure that there will be green branches on the tip of the branch.
Make sure your tools are sharp, and sterilized before each new cut.