Many projects are sick before they even begin. The cause often is an improper, indistinct clarification of what has to be done. Particularly in many cases the translation from business requirements into technical specifications is a weak point.
To understand the scope right you need to be clear about all stakeholders involved. Conduct thorough stakeholder analysis to assess interests, influences and requirements. Sometimes the scope needs to be developed iteratively from project definition/charter, e.g. by an agile approach or in design workshops.
Describe scope and requirements result-/deliverable-oriented and unmistakably, not descriptive. Define and agree to measurable success criteria. Generally you should ensure requirements’ sign-off before starting development to avoid later discussions on scope.
Only when your scope is “properly” clarified you can plan the execution “properly” and realistically!
Subdivide/show progress by multiple delivery sub-units fit to formal approval underway.
Always assess what you do, what you think you should do, and what your stakeholders think you should do!