Mine was a long time ago, but I'd say -
Expect it to be a slog at times, however much you love your subject. Expect that there will be times when you feel that your writing is crap and boring and you have nothing worthwhile to say. Just keep going.
There will always be one more piece of research to do, one more article to read. You have to say STOP at some point, or you'll never finish.
Write the introduction last. You can make notes and produce a rough draft as you go, but you won't necessarily know what should be in your introduction until you've written the rest of it.
Putting together the acknowledgements, chapter listing, lists of tables and illustrations, bibliography etc will take longer than you expect. Estimate how long you think it will take, then double it. (That said, technology has moved on hugely since I did mine, so I expect it's all far more straightforward now.)
Back it up. Back up everything you do. Do it at the end of each day, then do it again. Upload it to the Cloud, put it on a USB, e-mail it to yourself. When I was a postgrad student there were all kinds of horror stories circulating about disastrous things that had happened to the only copy of someone's thesis. Most of them were no doubt urban myths, but why take chances?
When you've finished and got your doctorate, it's the most tremendous sense of achievement. It's very satisfying to be able to call yourself 'Dr Pakkas'.
Good luck!