Help with transitioning a horse from pasture to stall?

In a week or so I'm going to be buying a horse and trailering her to my boarding stables. Right now she is kept in a pasture so she has full, all day access to grazing, along with being fed twice a day. Where I'm going to be keeping her, however, is a large paddock that has no grazing, so she'll only be fed twice a day, in the morning and evening. I know that I can't just change her eating habits from pasture to stall because of risk of colic caused by acid buildup....or something like that. I have a basic idea of how to transition her to twice a day feed in a stall but I'm not entirely certain. I've never had to do this with a horse before, so I'm a little confused. So - how should I go about this transition? Should I feed her at least 3 times a day at first, in a hay net so she eats slower? How long until she adjusts? How do I know when to start feeding her less often during the day? Should I buy a bale of gr/alfalfa mix so I can feed her more often myself? (The barn provides twice a day feeding @ no extra cost, would I need to supplement this?) Hopefully I don't have all my information wrong.....any tips are helpful, thanks so much! =)