Model the skill. Signal the preschoolers to have closed mouths, open ears and eyes on the teacher. Ask an open-ended question based on the topic or lesson being addressed, and think aloud your thought process and your answer to the question. The preschoolers need to witness the entire process - from hearing the question to considering it to answering it - in order to master the skill.
Practice with scaffolding. After the preschoolers watch you model how to answer open-ended questions, invite them to answer a different question with your support. Select a couple of students who are able to join you in thinking aloud about the question posed and how to answer it.
Check for understanding. Ask open-ended questions of a sampling of your preschoolers to determine if the skill was learned or needs re-teaching. For example, in a class of nine ask three or four of the kids questions. If the majority of your sample group can answer open-ended questions, the instruction was effective.