The idea that writing is a process is so commonplace these days, I hesitate even to mention it. But it must be mentioned, because in the writing center we see so many instances of classes where writing is not treated as a process. We see students struggling with assignments because they have been given no directions or suggestions as to how to go about doing them. There is much they don't know. For instance, in some kinds of writing, such as essays, the parts can be written in several different orders.
Writers can start at whatever point they find easiest and gradually work toward the hard parts.
They may write the introduction last of all, after they have explored what the paper is really about. After finishing the draft, they can often move parts around to find the most effective order. Other kinds of writing, such as sociological studies, must start with the introduction, which must present the question to be researched very clearly before the study can even be conducted. The discussion can't be written before the findings have been presented. The parts of the study build upon one another other.