For me, one of the coolest things about being a freelance game designer is that I get to take a step forward and fill a role that I normally manage in my other-worldly fulltime career. It lets be focus on one specific area and be creative in ways that are oddly joyful considering the limited scope. Much like a game, project management has a core set of rules, and certain pillars are generally adhered to. I'll relate them here as to how I might manage a gaming project.
1) Project Initiation - This is where you gather the requirements for your project. Here you might decide that you need a sourcebook on trolls and ogres. In the initiation phase you start to form the team of designers that will work on the project and also any incidentals, such as the project lead.
2) Project Planning - In the planning phase, after the incidentals of the initiation phase have been completed and approved, you begin to lay out how it will cost to get such a project off of group. You also start evaluating the associated risks involved with the project and any negative outcomes that might arise (risk planning). Also in this phase, you work to layout how the sourcebook will flow, giving a high-level view of the chapters and sections involved. You work to mold them into a cohesive book that would be appealing to your customer base.
3) Project Execution - This is where the pen meets the paper (as it were). Your team of designers are assigned different parts of the book and they are sent off with their lead to complete it. Milestones should be established and regular feedback given in regards to communication. This is also the phase where critical feedback is provided as change management is a crucial part of any project plan. The last thing you want is for this phase to be completed, only to have to come back to it again in the future. All portions of the project should be reviewed and amended, as appropriate, in this phase. By doing that, you ensure you mitigate anything major as the project moves into the final phase. Here is where game development will take place as well as some of the initial editing.
4) Project Closure - This is where you likely have the brunt of the final editing. It is here that you also finish your layout work as well as complete the tying off of loose ends.
Project management principles have been developed to fit in any repetitive process. They work well, and they should not be ignored. If you do ignore them, your production will suffer, your steak-holders frustrated, and your final project in disarray. Remember that it is always best to manage expectations and to keep communication as open as humanly possible. If you, as the project manager, find yourself having to go back, reorganize, and reassign duties found in the execution stage, you likely have failed to meet the criteria in the phases that came before.