Although this is about questions for discussions in a community, these are good basic guidelines for online learning discussions too. These could work as guidelines for a student-led discussion on how to write good questions.
Tips for online instructors (and IDs) on how to write questions for online discussions. Three types of questions are identified: factual, thought (Socratic), and problem-solving. By Judith Boettcher–this is now part of the book “The Online Teaching Survival Guide: Simple and Practical Pedagogical Tips”