Tools first - a pair of good bypass pruners, a sturdy and sound stepladder if needed
Pruning - Where you prune is where the new growth will resume. While you can prune to a given height, regrowth will occur from several inchs below your cut and in a short time be back to the previous height. Attempt to find a smaller twig/branch or bud to prune to and not pruned off leaving a stub. Once you begin pruning, you will likely find yourself pruning every year attempting to maintain growth. For example if the Arbs are naturally maturing at 12-15 feet and pruned down to 6-8 feet, you will always be pruning in an attempt to manage heights. Pruning away a few bits of the side branches to allow your deer fence to work properly is not like attempts to limit top growth.