Starting the Getting Things Done process.
Dedicate several hours of uninterrupted time. Gather everything possibly actionable into a physical “inbox” space. Write everything that comes to mind that needs doing onto separate pieces of paper. Once this capture process is reasonably done, being the clarification process one item at a time. If doing small items slows the process too much, consider shortening or suspending it. Put deferred items into a new stack, otherwise fully triage the item to the right place. Schedule a yearly purge day for reference materials.