What it's for
The plan is the contract you make with yourself when the market is closed. It decides in advance what you trade, when and how much you risk, so that during the session you only have to execute.
The sections of the plan
Markets and hours: which instruments and in which windows you trade. Setups: two or three configurations described with precise entry, stop and exit conditions. Risk: maximum percentage per trade, maximum daily, weekly and monthly loss.
Routine: what you do before the session (news, levels, watchlist) and after it (journal, screenshots, a score for the day). Review: when you reread the numbers and by what criterion you change a rule.
A template to copy
I trade EUR/USD and the DAX from 9 to 12. Two setups: a break of the first-hour high and a retest of the broken level. 0.5% risk per trade, stop after two losses or −1.5% on the day. Checklist before every entry. Review every Friday: I change at most one rule a week.
Make it binding
A plan locked away in a document is forgotten at the first bad trade. Copy it into your journal as rules and a checklist: it will warn you or stop you when you're about to make an exception.