I recommend fitting the prong collar on your dog so that the collar is high up on the dog's neck. The prong collar should be snuggly fit. The prong collar should be snuggly fit, and rest behind the dog’s jawline and ears. The prong collar should never be pressed over the dog’s face in order to put it on. If improperly applied the prongs of the collar could injure the dog’s eyes. When fitting the collar, you must pinch one link at the narrowest part of the collar, in order to release an individual link and open the prong collar. Then the collar is placed around the dog's neck, (think about how you would put on a necklace).
If the prong collar is too loose it can sag around the dog’s shoulders, which is ineffective when making a correction. Instead of an effective correction of pressure and release, the prongs rest into the dog’s shoulders contributing to constant state of correction (pressure). Being constantly corrected is confusing to the dog and will almost certainly slow the progress of any obedience training regimen. The key to good obedience training is to communicate clearly to the dog as to how their behavior can turn the pressure both on and off.
As a safety feature, I recommend connecting the prong collar to the dog's flat collar with a carabiner. This is a safety net of sorts. If the prong collar were to break away at any point, the flat collar will still be secured to the dog. The attached pictures should help to illustrate the safest most humane use of a prong collar.
For more information on how I perform obedience training with prong collars, please see my website: http://goodkarmatraining.com/training-your-dog/