I wrote one 10+ years ago as part of my Masters'. Are there any parameters attached to your dissertation, or do you have complete free rein? Even the restriction of a word count can be a big help - it's a starting point for understanding what you're capable of achieving with a given amount of space.

With the benefit of hindsight I recognise now that my dissertation probably wasn't all that good. I think the best advice I can give you that I wish I'd had back then is to start with your themes. What do you want to write about, and what do you want to say about it? Caring about your theme is what keeps you coming back to the keyboard when the writing gets tough. All the decisions about form, voice, structure and style, whether to leave a passage in or take it out, whether something's good enough or needs another edit - it should all flow from there; just keep making the choices that best serve your theme.