For most usual conifers, like Chamaecyparis, Thuja, Taxus, and Junipers, the best time is just before new growth starts between April and May.

Titchy asks about which tools to use for pruning..
Can you answer that part as well, please, Bluespruce. Thanks. :o)

Yes sorry, hand shears can be used, but don't cut back into old wood, just previous seasons growth.

blue spruce is right if you cut to far it wont grow back ever so thats quite importent. if its a hedge or shaped i would quite litteraly put up some markers for hight and width and cut anything of that comes past this as soon as you can within reasen.this way you will never accidently cut to much.it worked well for me anyway.