There are many different ways to put a broadhead on an arrow. Some ways will be safer than others. For one to understand how to put a broadhead on an arrow you must first understand the genealogy of said arrow.
Understanding the Arrow
Arrows are typically a long thin projectile object that is catapulted from the string of a bow. The non-fletched side of the arrow will have a threaded insert in which the broadhead will screw into the insert. The blades on a broadhead can be very sharp so it is very crucial to be careful when handling such objects. While the blades are very sharp it is also critical to be careful around the tip of the head as not handling with care could result in extreme bodily injury or even death.
Having the Right Tools
Most heads will come with a plastic and or metal tool usually comes in the package. Archers will use this to tighten the head down to the arrow. The wrench typically fits around the blades and ferrel of the broadhead turning clockwise on the arrows threaded bolt like inserts will tighten the sharp objects down within seconds. This being the safest most less devastating way to put a broadhead on an arrow but not necessarily the most common.
Some more daring folk will attempt to put these lethal devices on the arrow by using their fingers or other body parts. While this tactic could accomplish the goal of putting a broadhead on an arrow the injuries that could result may be detrimental to individuals involved. To tighten heads down you will need to apply a relatively large amount of foot pounds of pressure to the head once the threads inside the arrow and on the back of the head are maxed out. The twisting presureous motion one applies when twisting the broadhead significantly increases the likelihood of extreme injuries or even worse.
Once you understand the makeup of a broadhead, and the arrow you are working with, putting the broadhead on is not all that difficult. It is smartest and best to use the wrench supplied by broadhead companies to attach the head to the arrow. This will minimize the possibility of injury when handling these objects. While the tool may be the most efficient way to do so there are still some archers who are reckless enough to try to use their hands no matter the cost. Being one of these people is not recommended as the aftermath could be detrimental.