Bring together the people who have been doing the work and are collectively accountable. Also include any stakeholders who might have critical input or perspective to offer.
When the project plan is being created, the team leader/manager identifies points along the project schedule to conduct informal debriefs. Key milestones often represent good opportunities for a debrief meeting.
When possible, conduct the debrief as part of a regularly scheduled project team meeting or status meeting.
Identify the focus of the debrief (a recently completed milestone, etc.) and ask, “What did we want to happen, what happened, what did we learn, and how will we move forward?”
After the debrief meeting, make sure the learning is captured and distributed to those who can leverage the learning.