I recently got a big new set of Prismacolor pencils. (Please put in the comments if you want me to do a review of the pencils.) I learned from a recent art class that I took that when you are working with a new medium it is a good idea to start with black and white so you can practice working with your darkest dark and your lightest light. Value is a big part in making any picture look realistic.
Always start with making a value chart so that you can see what your medium is capable of.
Then you can take a look a what you want to draw. First looking for the darkest dark and lightest light.
Then you draw everything in between. Soon you will have your full image. You can then go through randomly and fix any details that you noticed that are wrong.
Of course, you won't get everything perfect but that is what practice is for.