If the project is voluminous and requires much time to be completed, you might need to start writing the text along with the research itself.
However, under any circumstances, you are recommended to start writing as early as possible to leave yourself enough time for revising, editing, proofreading, and other seemingly insignificant but very time-consuming things.
This method implies creating numerous scholarly articles that are suitable for publications in academic journals.
In other words, the research is conducted in small portions published one by one.
The subject of your research is a different thing; it’s a part of the object, like peculiarities or sides of the object that seem to be significant from the point of view of science. You can either give attention to all of them, or choose one that seems to be the most important or interesting to you.
Yet, it’s very important that when you get down to writing your postgraduate dissertation, you have a clear idea of what the subject and the object are.
The first one is very unconventional and not well treated in many institutions.
Nevertheless, you may try to discuss using it with your professor.
On the other hand, the writing process might be a marathon-like, i.e., long, tough, and exhausting.
You are advised to find own pace and try to strike a balance between doing too little and too much.