Build the Trigger Handler

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

1. trigger OpportunityTrigger on Opportunity (after update) { — exactly Module 24's "Avoiding Logic in Triggers" pattern.
2. new OpportunityTriggerHandler().afterUpdate(Trigger.new, Trigger.oldMap); — 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.
3. } — closes the trigger.

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.');
            }
        }
    }
}

1. public class OpportunityTriggerHandler { — the handler class.
2. public void afterUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) { — the method the trigger calls.
3. for (Opportunity opp : newOpportunities) { — loops over the updated records.
4. Opportunity oldOpp = oldMap.get(opp.Id); — this is Module 24's Trigger.oldMap lesson doing real work — looks up this specific record's prior version.
5. Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won'; — justWon distinguishes "actually changed into Closed Won" from "was already Closed Won and something unrelated changed."
6. Boolean justUnwon = opp.StageName != 'Closed Won' && oldOpp.StageName == 'Closed Won'; — justUnwon catches the reverse transition.
7. (blank line)
8. if (justWon) { — checks the first case.
9. System.debug('Opportunity ' + opp.Id + ' just became Closed Won.'); — logs it.
10. } — closes the if.
11. if (justUnwon) { — checks the second case.
12. System.debug('Opportunity ' + opp.Id + ' is no longer Closed Won.'); — logs it.
13. } — closes the if.
14. } — closes the loop.
15. } — closes the method.
16. } — closes the class.

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.