Become familiar with the magic realism genre and writing style by reading well-known authors. One of the most best-known works is "One Hundred Years of Solitude," by Gabriel Garcia Marquez.
Write a sentence that contains realistic elements, with characters and a setting that seem believable in the world that we live in. It can be a simple sentence with a character and some action, such as: "Jack knew that he would never make it to the movie on time with all the slow-moving traffic on the highway."
Add an element of magic realism into the sentence, but don't make it seem like it is a surprise or that it is out of place. You can continue to add magical elements as you develop a more well-rounded story. The sentence could look like this when it contains magic realism: "Jack knew that he would never make it to the movie on time when he saw that the cars were all moving too slowly through the thick sludge of pudding falling in thick raindrops."