We were told to call the hotel and ask for Guest Services to set up the appt.. I think it opens before the store opens to the general public and it costs $35 or $45, can't remember but whatever you pay to go, they give you back in a gift card to buy stuff there.
Here is what the Disney site says, "Contact the Guest Services Desk to book the fun in advance or book at check-in." When we did the early entry Build-a-Bear, we always booked it before we got there, up to a month ahead, I think.
From the schedule I have, it looks like they only have it on Sat and Sun at 8:30. If you do it, have fun and come back to tell us how it was!
Here is an old review I saw on another website(notice they called Vacation Planning for their appt):
"We did this for my niece. We called vacation planning and they schedule it for you. They let you in an hour before the store opens to the public. They have a little "workshop" where the speaker talks about their products and the various benefits of each potion. It's basically a skin care lesson. Then they direct you to various make-up artists and you get a makeover. While you wait for the artist and afterwards, you have time to roam around the store to buy what you want. They also give you a lot of free samples of the speaker's products. I can't remember very well but I think they came in a tied tissue paper gift for each of us. You were free to ask questions about the different products that the speaker was pushing and the makeover was a great way to start the day! Some people don't like too much makeup. Just let the artist know to keep it simple."