Build the Trigger Handler

trigger OpportunityTrigger on Opportunity (after update) {
    new OpportunityTriggerHandler().afterUpdate(Trigger.new, Trigger.oldMap);
}

Exactly Module 24's "Avoiding Logic in Triggers" pattern — the trigger itself does nothing but hand off to the handler, passing both Trigger.new and Trigger.oldMap since this requirement needs to compare old and new values.

public class OpportunityTriggerHandler {
    public void afterUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) {
        for (Opportunity opp : newOpportunities) {
            Opportunity oldOpp = oldMap.get(opp.Id);
            Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won';
            Boolean justUnwon = opp.StageName != 'Closed Won' && oldOpp.StageName == 'Closed Won';

            if (justWon) {
                System.debug('Opportunity ' + opp.Id + ' just became Closed Won.');
            }
            if (justUnwon) {
                System.debug('Opportunity ' + opp.Id + ' is no longer Closed Won.');
            }
        }
    }
}

This is Module 24's Trigger.oldMap lesson doing real work: justWon and justUnwon distinguish "actually changed into/out of Closed Won" from "was already Closed Won and something unrelated changed" — the exact comparison this requirement depends on.

Without comparing against oldMap, every update on an Opportunity already sitting at Closed Won (say, just updating its description) would look identical to one that just became Closed Won — double-counting the same Amount into the rollup every time the record is touched. Catching only the actual transition is what keeps the rollup correct over time.