The tasks need to be scheduled in the correct order and dependencies between tasks factored in. In complex projects several tasks will be performed in parallel to maximise the total project time. The plan will also take account of the project budget.
It is probably a rare project that is 100% successful but whatever the final outcome of the project always aim to find some element that was a success and make sure you reflect positively on the successful parts. Hold team reviews to learn from both the successes and the mistakes and take the time to write a report documenting any remaining issues that need to be followed up.