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Step one: define the line and who joins it
Decide exactly what the queue covers. Take a returns counter in a hardware store. One line might serve returns only, while special orders form another. Write down how people join: scanning a code at the counter, texting a number or being added by staff. Set a cap, perhaps thirty people, so the estimate stays honest. Also decide what information is needed. A first name and mobile number are usually enough. Collecting less makes joining faster and reduces what you must protect. Keep the first version to one line and one location until staff are comfortable.
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Step two: set the estimate and the call-out
Estimate waits from real handling times, not hopes. In the returns example, time ten ordinary transactions across a normal afternoon and use the average, then adjust it after a week. Choose when to alert people: perhaps when three customers remain ahead, giving time to walk back from the aisles. Write the message in plain words with the position, the expected wait and a way to leave the line. Avoid promising exact minutes. A range such as ten to fifteen minutes builds more trust and creates fewer disputes at the counter when someone arrives early.
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Step three: handle missed turns and review
Agree what happens when someone does not appear. A common rule is to call twice, wait two minutes, then move that person back one or two places rather than deleting them. Tell staff who can override the order, such as for accessibility needs, and record each override. After the first week, review three numbers: average actual wait, number of missed turns and how often staff skipped the system. In the hardware store example, frequent skips usually mean the estimate is wrong or the alert arrives too late. Adjust one rule at a time so you can see what helped.