Anthony Boobier created the Facilitation Canvas as a tool to help facilitators prepare to run a session. He uses it as part of their ICAgile team facilitator course. The canvas is made up of 8 boxes, each with an associated set of prompt questions that I have found work well as a guide when filling it out.
Read all about it here: https://medium.com/path-pattern/the-retrospective-a-known-recipe-for-improvement-with-my-own-spice-da534c6628cc