Wash and dry your hands before applying any dressing, and wear disposable surgical gloves if you have them.
Also:
sit or lie down, or get the person who's bleeding to do this
if you're helping someone else, tell them what you're doing as you apply the dressing
stop any bleeding by applying pressure and raising the affected leg or arm higher than the heart
use a dressing that's slightly bigger than the wound you want it to cover
hold the dressing at the edges, keeping your fingers away from the part that's going to cover the wound