The planning of a project must be based on several questions, such as the following: what do we need to make it? How do we do what we need to do? Who will do it? When is the deadline? How much will it cost? What will it achieve?
It is interesting to note that, in most projects, planning actually begins even before you start the project itself (as one of my colleagues would say, the project before the project) and would not be completed even if the project itself has formally been handed over and has performed all the necessary post-mortem actions.
Each individual is responsible for each stage of the project. If your project planning is of poor quality, or you go into it with a complacent attitude, it is likely that your project will finish in the infamous 78% of projects that are started but never finished.