Q. How should I discipline my puppy when an "accident" occurs?
A. The puppy must be disciplined as soon as the mistake has been made, when the incident is still fresh. Reprimands made for accidents that happened some time ago are futile at best, and harmful if done improperly. If you catch the puppy as he is making the mess, or is about to make a mess, discipline him in a firm way without being too severe by using startle techniques that will cause him to immediately stop what he is doing. A strongly voiced "No" or "Stop", a sudden loud noise to catch his attention, or taking hold of the scruff of the neck are all good ways of turning the puppy's attention from what he is doing and toward you.
Follow all of these actions by immediately taking the puppy outside to finish relieving himself and reinforce the "good" behavior (going outside) with rewards such as patting, verbal praise, and/or training treats.