Start slowly, with only a few prepositions. Review a basic and simple sentence structure to give students a model for using prepositions. Ask questions to build the example sentence. Point to a person and ask "Who is she?" Write the student's name -- Sally, for this example -- on the board, then ask "What is she on?", emphasizing "on" when you ask the question. Write "chair" beside her name. Then complete the sentence: "Sally is on the chair." This gives the students the basic sentence structure to work with prepositions. Leave the sentence on the board so they can refer to it. With adult learners, you can use words like "subject" and "verb" and expect them to understand.
Write three or four simple prepositions on the board in your classroom, with lots of space between them: on, next to, and behind, for example. Then, referring to the basic sentence "Sally is on the chair," ask Sally to stand up and another student to sit down in her chair; then ask how the sentence might change. Then ask Sally to stand beside the chair and ask which word changes. Repeat this process with a few different students or objects from around the room; then change the objects around to practice the propositions.
Add a few more propositions to your list on the board. As an ESL teacher, you sometimes have to make wild and elaborate gestures to emphasize the meaning of words. You will be testing in this regard when explaining the difference between "in" and "on," which may seem basic to you but which confuses many ESL students. Make sure to have an item in the classroom that you can put something into to practice with "in."
Play games, especially with an ESL class of young children, to review. "Simon Says" is great for reviewing prepositions, as you can give directions for students to move around, like "Simon Says stand on your chair" or "Simon Says get under your desk." Students respond well to the movement and use the prepositions as if they were second nature after a while. With adults, review games can be a bit more difficult to make engaging, but try to bring out the competitive side of your students by playing a timed review game. Divide the class into teams, then tell students they have one or two minutes to write sentences about objects in the classroom using as many prepositions as possible.