Project planning within project management is the process to quantify the amount of time and budget a project will cost. The purpose of project planning is creating a project plan that a project manager can use to track the progress of his team.
Make an inventory of all the work that needs to be done with an estimate of the time it will take to complete by a single team member. This can be done in a planning session with all the team members. Tasks that will take over three weeks to complete need to be broken down further to get good granularity. To avoid getting swamped with details, the tasks at the lowest level should take approximately 1 week. The result is a work breakdown structure. Make sure that having the project's deliverables injected into the organization or its environment will actually cause the expected benefits (project objectives) to materialize.
Identify the resources needed to complete each terminal element of the WBS. At this point you can usually estimate the cost to deliver each terminal element and, consequently, the project (bottom-up approach). Sometimes a top-down approach to estimating costs is also possible by means of using coefficients (e.g. it costs between $X and $Y to build a square meter of a house of such-and-such a standard).
Make a decision whether this initial plan makes sense, i.e. whether the costs justify the benefits. Modify the objectives and the supporting work as necessary.
Define dependencies among tasks. Some tasks need to be completed before other tasks can begin. By putting tasks into their relative completion order, a project manager constructs a project network.
Calculate the minimum time the project will take: it is the longest path through the project network from the start of the project until its end. This path is called the critical path (or critical chain, if resource dependencies are taken into account). Other tasks can be done in parallel to the critical path but any delay in the tasks on the critical path will automatically result in a delay in the overall deadline of the project.
Create a project schedule (e.g. in the form of a Gantt chart).
Plan for risk management and modify the project plan accordingly.
Commit the organization to starting the project implementation.
Project planning is not something that is done only once at the beginning of the project. It should be an ongoing task of the project manager to keep an eye on the progress of his team and update the project plan accordingly. Project management software can be helpful if used properly. There are several project management standards that describe in detail how to plan and manage a project.