Drawing a picture will really really help you to be able to visualize it. Basically, in internal respiration, oxygen dissociates from oxyhemoglobin and diffuses into tissue cells. Carbon dioxide is going in the opposite direction, from the tissue cells into systemic capillary blood. Some of the carbon dioxide binds with hemoglobin to form carbamino hemoglobin, while some carbon dioxide combines with water to form carbonic acid. This carbonic acid then dissociates into bicarbonate ions and hydrogen ions. The bicarbonate ion leaves the red blood cell and goes into the plasma and in order to maintain the electrical balance, a chloride ion enters the red blood cell (since both have a negative charge). This is the chloride shift.
In external respiration, when carbon dioxide is exhaled, hemoglobin inside the red blood cells unload carbon dioxide and pick up oxygen from alveolar air. As oxygen combines with hemoglobin, thi scauses hydrogen ions to be released from hemoglobin. Bicarbonate ions from the plasma come into the red blood cell and bind with the hydrogen ions to form carbonic acid. This carbonic acid then dissociates into water and carbon dioxide and carbon dioxide diffuses from the blood into alveolar air so that it can be exhaled. But when the bicarbonate ion enters the red blood cell to bind with the hydrogen ion, a chloride ion must leave the red blood cell in order to maintain electrical balance. This is the reverse chloride shift and it is basically a reversal of everything that happened in internal respiration with the chloride shift.
Hope this helps some.