To treat a pneumothorax in a dog, a procedure called thoracentesis is performed to remove the accumulated air from the pleural space. A needle or catheter is inserted into the chest cavity to aspirate the air during thoracentesis. The exact location where the needle or catheter is inserted depends on the size and location of the pneumothorax. Typically, the procedure is performed between the ribs in the intercostal space. The veterinarian will determine the specific site based on their assessment of the dog's condition and imaging findings. It is important to note that thoracentesis is a procedure that a veterinarian should perform, as it requires skill and knowledge to avoid complications and ensure the dog's safety.