Question
can you please tell me when is the best time to prune my arbovitees

what is the best way to do so
                               Thank you

Answer
Generally you should not prune MOST arborvitae at all except to limit height. I would not recommend it. But if you did, same as most trees and plants the best time is during a dormant period such as late fall or early spring.
See these links.
http://www.gardenersnet.com/tree/arborvitae.htm
http://www.aboutarborvitae.com/pruning_arborvitae.shtml
http://www.ubcbotanicalgarden.org/forums/showthread.php?t=6800