A good way to pick a specific project within a broader area of interest is to find some potential supervisors whose groups are within your desired field, look through their recent publications in the area and their group homepages and see what takes your fancy.
For example, say I am interested in doing an MSc in inorganic catalysis, but don't really know what specifically there is that I could study in that area.
First I go onto my alma mater's homepage (or another desired destination) and see if there are any groups in the Chemistry Department who are interested in inorganic catalysts.
Next I see specifically what particular aspect of the catalysts they are involved in, be it synthesis, computational design, toxicity, etc. I look at their recent publications, if one title catches my eye, or particular paper is really cool, I add that group to my 'possible projects' list. Do this until you have 2-4 possible supervisors/projects. You might even get an idea for a project on reading one of these papers, which you can say you wish to discuss when you contact them.
Then I contact the heads of the groups directly, detailing briefly who I am, something along the lines of:
'I am writing to you to ask if you are currently accepting graduate students because I wish to pursue my interest in inorganic catalysis by applying to the University of blah to undertake a MSc in your group.
I am a recent graduate of blah with a 1st/2.1/2.2/rubber duck/etc. I attach my CV but briefly --short list of most notable and relevant achievements if any--
I recently read your group's paper on blahblah --something that indicates you actually read the paper and thought about it-- and have some ideas for a project I would be interested in discussing with you.
Grateful for the opportunity to explore further possibility of working in your group --propose contact options, indicate if possible that you would be willing to visit, this being the best way to sell yourself and find out a bit more about the group to see if you think it would be a good fit: interviews are a two-way process
With respect to what your project will look like, that doesn't need to be much more than a rough proposal until you actually have a place, after which you will need to write (possibly with your supervisor's help) what essentially amounts to a mission statement for what you are setting out to do, why you want to do it/what you think could be gained. More specific guidelines will probably be available on the Department or University homepages.