The point is to make the important decisions before the market is moving. Typical contents are a check on sleep and energy, a look at scheduled events, the levels or conditions that would qualify a trade, the maximum risk for the day and the single rule you most need to keep.
It works partly as a filter. If the check shows you are exhausted, rushed or angry, you can reduce size or skip the session while the cost of doing so is small. Decisions made after the first loss are made under pressure and tend to be worse.
Keep it short enough to do every day. A routine that takes an hour is abandoned within a week; one that takes ten minutes survives. Writing the answers down, even in two lines, creates a record you can compare with what happened.
A useful addition is a written if-then plan for your costliest habit, for example: if I take two wins in a row, then the next trade is risked at the planned amount. Reading it before the session puts the cue in mind when it appears.
A routine cannot guarantee good decisions, but it removes avoidable ones from the session.