A plan turns discretion into decisions made in advance. At minimum it defines the markets and sessions you trade, the setups you accept and their entry, stop and exit rules, the risk per trade, and the limits that end a day or week, such as a maximum loss in R.
Its value shows when you are under pressure. After a loss or a win streak, judgement is least reliable, and a written rule is the one thing that has not been affected by the last result. A plan that exists only in your head gets edited by your mood.
Specificity decides whether it works. A rule must be checkable: I risk 1% per trade can be marked yes or no, while I manage risk carefully cannot. Checkable rules also make a discipline score possible.
Plans should change, but on a schedule and for evidence. Review it monthly against your record, change one thing at a time, and write down why. Changing rules in the middle of a session is the behaviour the plan is there to prevent.
Keep it short enough to read before every session. One page that you use beats ten pages you filed.