To be a good shapeshifter, you must master the art of observation. It is impossible to shapeshift into an animal's body, even in the mind, if you do not know how the body works. If you live in an area where you can observe animals in their natural habitat, you can start the process of learning to imitate them. Go to the zoo, or a nearby park or forest. Observe how the animal moves. The way it moves its ears. Watch the muscles flex as it walks. Observe the pattern of the wings moving. Take notes. Mimic the movements and sounds. Read up on the animals and learn their ways. Begin to think yourself in the part of the animal. Think how it is to be the animal.
You can find more information on Shape-Shifting in my book, Celtic shamanism: A Contemporary Guide for the Beginner.