Featured guides are handpicked by a community manager. There's no real criteria for a guide to be featured. If your guide is considered good, it will probably end up featured (rotation changes weekly if I remember correctly).
There's also a chance that your guide has been featured already in previous rotations.
Close, I try to follow the free week as much as possible. If it happens that I can't find 3 solid guides to feature though, sometimes I add a guide or two outside the free week.
There's also a chance that your guide has been featured already in previous rotations.