See the Sketches J.R.R. Tolkien Used to Build Middle-Earth
"The simple answer is that he wrote it...The more complicated answer is that in addition to writing the story, he drew it. The many maps and sketches he made while drafting The Lord of the Rings informed his storytelling, allowing him to test narrative ideas and illustrate scenes he needed to capture in words. For Tolkien, the art of writing and the art of drawing were inextricably intertwined.
In the book The Art of The Lord of the Rings, we see how, and why."