The transformation of your composition from 2D to 3D will involve many iterative procedures. A good 3D composition must be carefully planned and designed – with the application of correct design principles and methods, your process of design must be clearly illustrated to assist you in both the process of designing and presenting your design.
Here are some examples of diagrams that clearly illustrates the iterative process involved, how the 3D massing were made and how do they transform into becoming a composition. However, these are samples of diagrams that were used on various different contexts. You are required to develop your own diagrams based on a solid understanding of your own design processes.
Identify your own operative techniques and processes, understand the design principles that your use and formulate a technique in representing and communicating them. Do not simply copy what are being posted here. A good diagram comes from a good design understanding that originated from a good design concept and intention.
Also remember that diagrams can also come in the form of physical 3D objects. Your sketch/study models also can act as a good and informative diagram to explain the process of your composition design. Think of ways to utilize your study models as diagrams.