Check out this great post from The Honeybee Conservancy on how smoking works and how to use a bee smoker.
My experience with smoking bees is always have it available even if you don’t need it. Use it judiciously, and sometimes not at all. In the spring when the bees have “nectar on the brain”, it may make little difference. In these instances even a little sprayed on sugar water can be used in place of smoke! (Never use sugar water spray during a dearth – after the main nectar flow is over or robbing will result!) This is not the smoker I actually use, but it is a way of saying “Less is more.”