What length papers are you used to writing? It might help to think of it as a series of papers that length rather than one long document.

If you think of about how many parts / papers you would do (chapters if you like). Then you can break those parts down again into sections (of about 2 pages each) and then... You are kind of good to go. Save chapter one for some kind of introduction.

I am assuming here that you are an arts / social sciences type rather than needing to write up an experiment...

The best advice that I've been given is to think of it as a google map with different levels of zoom.

1) The contents page. Headings for the different 'chapters' you are going to write (helps to start with this being a manageable paper length for you and then it can of course evolve differently).

2) The section headings within each chapter.

3) The paragraphs that fall under the section headings.

Start by the furtherest level of zoom. The contents page. Then try and work out some provisional sections and then (when something seems interesting to you) have a go at writing the sections. Getting stuck on the paragraphs? Then zoom out a little to the section headings and play with those for a while.

Your advisor may be one of those advisors who isn't so happy to TALK about your work he would rather SEE it. Instead of telling him you are stuck the way to approach it might be to give him some work in progress. Whether it be the provisional table of contents (maybe with the section headings too) or a few sections or a chapter. Once he has some idea of what you are up to then he is better placed to help you improve it.

Good luck.