How do we create a trigger handler in Salesforce?

Hi,
trigger CaseLegalUpdate on Legal_Description__c (after insert, after update) {
List<Id> legalId = new List<Id>();
for(Legal_Description__c lg : Trigger.new)
{
if(lg.Case__c!=NULL)
{
legalId.add(lg.Case__c);
}
}
List<Case> caseRecord = new List<Case>();
if(legalId!=NULL && legalId.size()>0)
{
for(Case c:[SELECT id,Check_Legal_Description__c,RecordType.Name FROM Case WHERE id IN:legalId])
{
c.Check_Legal_Description__c=true;
caseRecord.add(c);
}
update caseRecord;
}
}