You're correct, you determine the winners of a shootout, send posses home, and then resolve the job's effects-
If the leader’s posse wins the shootout, or no player forms a defending posse, the job succeeds. Otherwise, the job fails. After the shootout ends, regardless of the outcome, survivors in the leader’s posse go home booted. Surviving dudes in the defending posse stay where they are. Now resolve the effects of a successful job as given on the card, assuming the mark is still in play. If not, the job succeeds anyway but the mark is unaffected.