First, here is Linda Burkhart's website with her handouts from trainings she had done.  

She posted her 71 page handout from the two day preconference she did at ATIA this year.  It is a valuable resource to look to for a variety of AAC related questions.  It's a ridiculous amount of information, but answers so many questions.  She does briefly go over her Stepping Stones switch access process in this handout.  

Also on her handouts page is a very good in-depth article about Stepping Stones, which was published in  ASHA Sig 12 (AAC) Perspectives newsletter.  The stepping stones process is what I use when teaching switch skills.  

The last resource I have for you is a video of presentation that Linda and Gayle did for the Angelman Syndrome conference.  (The video wasn't cuing up right when I linked it, but she starts talking about modeling at 57:40.)  Linda talks about modeling using the child's system "some of the time" and using direct selection other times.  You'll also find some very helpful resources in all of the links about how to use aided language.  I find self-talk to be so helpful when working with these individuals who use an alternate means to access.  I speak out loud my thought process while the scanning is going on, not all of the time, but definitely some of the time so that the individual can hear what I'm thinking and how I make that decision to activate.  (This is also on her handouts website in a couple different parts, so if you want to watch the whole presentation, go to her website.