Here are a few useful tips from when I wrote my PhD thesis:
The most important thing to do is just write something. To explain, what you write doesn't need to be perfect, you don't need perfect prose or spelling or grammar or whatever, you just need to have something - anything - written down. Then, later on you can revise and improve it. For example, if you write 500 words of word salad, it will still contain some pearls and nuggets of wisdom, so later on you can trim the fat and end up with say 200 words of top quality text.
Set yourself a target of words/pages written for each day - even for each hour. For example, I set myself a goal of writing at least two pages a day, or about 200 words per hour. Again, later on you can revise and improve it, but if you do this then each day you will end up with around 1 page of good quality material. If you don't set yourself a target and just think "Ok I have weeks of time, I am sure by the end I will have written something" then you'll be shocked by the end to realise you've written far less than you hoped. You'll procrastinate and be inefficient. Set targets for each hour/day and don't take a break until you've met those targets. Pretty soon you'll realise you're only punishing yourself by procrastinating and just knuckle down and write.
A lot of stuff you have written throughout your PhD may still be useful. For example, internal reports for your supervisors, or abstracts for poster sessions - even the content on the poster - and most importantly large chunks of any papers you've published. Probably about 50% of the text of my thesis was recycled from previous stuff I'd written.
Certain parts of your thesis are pretty formulaic, e.g. your introduction, your conclusion etc. For these sections it's basically just filling in the blanks of a template. The harder parts are things like the motivation, the methodology, the results etc. So don't spend all your time on the "easier" parts, just write them and be done with them. Move on to them more important parts.