There are a hundred books about how to hold an effective retrospective, which I am sure are worthwhile reading. But in my role I usually do not host a project retrospective myself, so the following are my short notes as a kick start for the future:

Roughly, a retrospective consists of the following phases:

• Planning, before the retrospective: what is the goal of the retrospective? Identify and invite the fitting participants accordingly. Try to keep the number of participants low, if possible.
• Introduction: Make sure to explain why some participants are prsent - e.g. someone from management. Set the right mindset by emphasizing that the basic assumption should be that no one acted in malice and tried to do their best given available time, budget, skills and external constraints. Mention the goal: for me, personal and organizational growth for the future, or: make the whole project experience less stressful for everyone.
• Recapitulate facts: try to get people to remember the project and the context. Depending on the group of participants and everyones involvement in the project over time it may make sense to give a very brief project 'introduction'/reminder. A rough timeline of project milestones (start, end) or other important events should be prepared or created with the team during the event. The whole goal here is to refresh everyones memory for the next step:
• Evaluate: everyone writes notes about things that went well and things that did not go well. Afterwards, the team quickly groups the items to similar topics. Now the team prioritizes the topics: every participant gets N votes. The N most voted topics are discussed.
• Derive actions: the N most voted topics are discussed. The goal is to come up with actionable items: keep doing X, stop doing Y, start doing Z
• Make sure everyone gets a copy of that 'future work' report.