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Purpose: You have to plan before you start using a planning tool. A planning tool like MSP can help you in your planning, but it cannot replace you. You have to supply the information (list of tasks), so that the tool can do scheduling calculations for you.
You should start by creating a detailed list of project tasks that you need to schedule. Otherwise, you will miss things that will cause trouble later. It is difficult to insert a new task in the middle of a running project because it can disturb the whole schedule.
If you do not know all the tasks at the beginning of a project, then you should list higher level tasks and decompose them later. See also the following tip.