I would sincerely hope that after 6 years of employment she would accept with good grace that it is time for you to spread your wings and move on.
I would definately tell her the truth and make sure you keep even and steady eye contact.
I always think to myself - who are you (or anyone) that I have to lie to you.
Offer your notice period and if she wants you to leave straight away tell her that you understand why (although you must have had at least the last month to start purloining her clients if you wanted to!).
I would also thank her for the last six years and for everything you have learned along the way.
If they are all mean to you after this, keep your chin up and continue to act professionally and stay polite .. cry when you get home ok my lovely then dry your eyes and think that in one month's time you'll be gone and working for yourself!!
And good luck with your new business.
Jacqui xx