Ask yourself the question about the projects which you have experience of: “are the same problems and mistakes often repeated across multiple projects?” If the answer is yes, then applying this principle can help you avoid the mistakes from the past.
PRINCE2 recommends that previous projects are assessed for any lessons that can be applied on the current project even before the current project is initiated. When thinking of lessons, it is useful to consider what things went well and what things went badly from previous projects. These are captured and are incorporated when planning and developing strategies for any new project.
Lessons may be learned during a project which might be useful on the current project or later projects and so must be captured and incorporated into any later plans or strategies.
At the end of a project all the lessons learned can be reported as part of an End Project Report which can then be passed onto later projects.