Now is when you discover any misunderstandings or shortcomings in the informal agreements. It takes time to fix these. Students for whom english is not the mother tongue may have difficulty in getting ideas across, so that numerous revisions are required. And, truth be known, supervisors are sometimes not quick at reviewing and returning drafts. A rush job has painful consequences at the defence.
Tips Always keep the reader's backgrounds in mind.
It sometimes helps to mentally picture a real person that you know who has the appropriate background, and to imagine that you are explaining your ideas directly to that person. This is fundamentally important. Choose section titles and wordings to clearly give them this information. The harder they have to work to ferret out your problem, your defence of the problem, your answer to the problem, your conclusions and contributions, the worse mood they will be in, and the more likely that your thesis will need major revisions. Spell things out carefully, highlight important parts by appropriate titles etc.