Select a text appropriate for your age and reading level. Read each sentence slowly. As you read, use movies, television shows, photographs, pictures, paintings, people you have met, places you have visited and other experiences to picture what the writer is talking about.
Write, "When I read this I see" after each paragraph and follow with a simple sketch including the images you have summoned up as you read the paragraph. Alternatively, say the words out loud and describe out loud what you see after each paragraph.
Look up words you don't know, and things and places you have never heard of, to help with your visualization. For example, if you're reading about medieval times but don't know what a vambrace refers to, you can't visualize it. Looking up an image helps you visualize it and enhances your comprehension.