I wound down into it, so before my 'start date' I'd had:
week one of eating kind of normally but with fewer carbs, very low fat and no snacks
week two of the same but managing to do it more, plus getting really into water and quitting caffeine
week three I started.
So by the time I started it wasn't so daunting, and it didn't hurt. Really glad I did it this way - it was quite easy to start on a wednesday (I'd picked up the products on the monday).
and by then I'd planned what I was going to have when, etc etc.
Good luck with however you do it - the first few days can feel difficult whenever you choose - I guess it depends on your home life/work life/support/privacy etc. I don't have anyone at work knowing that I'm doing this - wouldn't even be a normal conversation to have with them anyway - and then at home I never shut up about it - OH is probably sick of it! bless him.
Let us know what you choose and how it goes for you xx