How did they discover Narnia in the Lion the Witch and the Wardrobe?

Storywise, the world of Narnia came into creation in 1900. The Pevensies enter it around 1943. C.S. Lewis wrote "Wardrobe" in 1950. All seven books were finished in 1952, however, Lewis only released one per year.

Lewis died on November 22, 1963.