The short answer
01 · The source
Who needs the result? What will they use it for? What must it hold? Split what it must have from what you would like to add. Do not drop safety, accuracy or agreed duties to make the job smaller.
02 · The size
First decide whether the project still has a point. If it does, name the smallest whole version that meets it. Not the biggest one you can picture.
03 · The move
Pick what you will hand over, who will review it, and a real date. Ask for the feedback you need before the handover. Do not wait until you have polished every last part.
Name what done means
Who needs the result? What will they use it for? What must it hold? Split what it must have from what you would like to add. Do not drop safety, accuracy or agreed duties to make the job smaller.
List the steps left and the choice that is blocking them. A project can look unfinished because nobody agreed who signs it off.
Keep a second list for later ideas. Then a new idea does not turn into one more thing this version must do.
One case
Say a guide keeps growing because you keep adding chapters. If the reader needs help with one task, a short guide that explains that task well may be enough. The extra chapters become a separate call.
Make the final agreement
Pick what you will hand over, who will review it, and a real date. Ask for the feedback you need before the handover. Do not wait until you have polished every last part.
Once it is out, write down what sits outside this version. Closing a project does not ban later work. If the point no longer holds, a clear choice to stop beats leaving it half open for ever.