On a project, the output of one task leads to the next task. The higher the quality of the output from the task the easier the next task.
Two key tasks at the beginning of a project lead to all other tasks. These are the scope and the high-level business requirements.
The scope lays out exactly what the project is focused on. The clearer the scope the easier it is to say if subsequent tasks are required and what complete looks like for those tasks.
The business requirements lay out the value or the why’s for business taking on the project. Everything after the business requirements are complete should relate back to those requirements. Either the task is furthering those requirements or the task is not required
Having these two critical tasks accomplished early in the project with a high degree of quality will make the rest of the project easier.