You can use pruning shears or trimming shears or electric hedge trimmers. Always wear goggles, a mask or bandana over your mouth is also recommended.

Beyond all the leaves is a section of barren stems, if you go that deep then you've gone too far. You want it to be wide in equal directions from its base. Hedges can be trimmed any time of year if they get enough water, even if they become stressed they can usually bounce back all on their own after a period of dormancy.