Having a clear mental picture of what you want your hands to carve, is essential to a successful wood carving attempt. How you get that mental picture depends on your past expertise in the subject matter. Even if you have a good mental picture of your subject matter, it is a good idea to have a picture or drawing handy during the wood carving process as a reference. Your hands can only carve what your mind can see.
A very valuable source for pictures and drawings of your subject matter is Google or Bing images. There are a ton of images available about all types of subjects. Next time that you have an idea for a wood carving project, try searching for images of it. You may even find changes that you want to make to what you had in your head, before you start carving the wood.
There probably is a very good chance that your mental picture of your carving subject is not as complete as a hardcopy image of the exact same subject. Small details that your mind may omit can make the difference in a good wood carving and a great wood carving.