Make a schedule. One approach that works for many people is to use a reverse calendar, where you plan your writing schedule from the due date and work backwards. If you know how much time you have to complete the project and break it up into manageable parts with individual due dates (whether these due dates are simply for you or if they are for your committee chair as well), you'll be less likely to get overwhelmed by the scale of the project.
Write a little every day. Writing 30 finished pages in two weeks is a daunting task, but if you write 500 words every day, then you will be able to meet that deadline with ease. Try not to get frustrated and put off your work because then it will pile up and become unmanageable.
Try the Pomodoro Technique. Many people who have trouble motivating themselves and being productive with their theses find it useful to work in “tomatoes” using the Pomodoro Technique. The basic idea is that you complete 25 minutes of completely focused work, then you get a 5 minute break. This breaks your work into manageable chunks and can cut down on the feeling of being overwhelmed that often accompanies a large, long-term project.
Take breaks. It is important, especially when working on a large-scale project, to give your brain a break every now and then. You can't stay focused and on-task 100% of the time without losing content quality, and letting yourself step away from your ideas for a couple days will give you fresh eyes when you come back to your work. You'll catch mistakes you didn't see before and come up with new answers you couldn't think of before.
Find a writing time that works for you. Some people work best in the morning, while others are able to focus more effectively at night. If you are unsure of when you are most productive, try different approaches and see what seems to work the best for you.
Write your introduction. You may find that your thesis proposal is a useful jumping off point for writing your introduction. You might want to copy and paste sections of your proposal for the start of your introduction, but remember that it’s okay to change your ideas as they progress. You may want to revisit and revise your introduction at several points throughout your writing process, perhaps even each time you finish a large section or chapter.
- If you do not already have a review of literature written, it’s time to do you research! The review of literature is essentially a summary of all of the existing scholarship about your topic with plenty of direct quotations from the primary and secondary sources that you’re referencing.
Contextualize your work. After reviewing the existing scholarship, you should explain how your work contributes to the existing scholarship—in other words, you’re explaining what you are adding to the field with your work.
Write your thesis. The remainder of the thesis varies greatly by field. A science-based thesis will involve few secondary sources as the remainder of your work will involve describing and presenting the results of a study. A literary thesis, on the other hand, will likely continue to cite secondary scholarship as it builds an analysis or reading of a particular text or texts.
Write a powerful conclusion. Your conclusion should details the importance of this Master's thesis to the subject community, and may suggest the direction that future researchers might follow to continue with relevant information on the subject.
Add supplemental information. Be sure to include relevant charts, graphs, and figure as appropriate. You may also need to add appendices at the end of your work that are germane to your work but tangential to the central question of your Master's thesis. Be sure that all aspects of your work are formatted in accordance with the guidelines of your institutional and discipline expectations.
Copyright laws vary by country, so this answer may be UK specific.
To be safe rather than sorry, it probably is a good idea to copyright clear third party works, especially if your dissertation will eventually be uploaded to an online depository, which is becoming more common. Imperial College London, as an example, specify that proof of permission to include third party works needs to be included in the electronic copy of the thesis and this may be a policy at other universities too. In addition, the source needs to be carefully referenced in a note to the figure. Not doing this can cause unwanted delays in depositing the thesis in the archive.
In my case, I found it rapid and free to include single figures from published journal articles in the thesis. Just as F'x says, requesting permissions can be done in a day, although it's probably best not to leave it to the last minute. I was sent through to Copyright Clearance Center's Rightslink from the published journal articles each time I requested permission and the process was straightforward.