When putting on fetlock boots, as with all other horse boots, make sure that the side fasteners are always on the outside. This prevents your horse from accidentally opening the boots and losing them. If the fasteners are on the inside, they could also get caught in each other and your horse could stumble.
The boots must fit snugly on the fetlock head in order to protect it properly, and the carpal joint must always remain free. The fasteners, whether Velcro or buckles, should not be tightened too much so as not to cause friction or pain, nor should they be too loosely fastened, as that could cause slipping.