Desensitization is a process of exposing a dog to a trigger by starting at a very low level of intensity or at a great distance and building up very slowly. It should be systematic, which means you have a plan to build up gradually. At every step of the way, your dog should be happy and comfortable and not overly aroused.
If your dog starts showing body language that he/she is fearful or reactive, you need to immediately go back to an easier stage of the plan. The obvious signs would be barking, growling, lunging, or escape attempts. Less obvious would be raised hair on dog’s back, hard stare, dropped tail or raised tail, stiff leg movements, ears sharply forward or plastered to dog’s head. If you jump ahead too fast and frighten or agitate your dog, then you will have slower progress or possibly make the reaction worse.
Over time, you will gradually increase your dog’s exposure to the trigger that causes his reaction, keeping his response under threshold.