I use to use a whole bunch of different foods and a more "dirt like" medium, but have switched over to the more simple method of yeast, charcoal (and or LECA), and water (very little). I add food at least once a week... but sometime twice a week. Depends on how much the springtails eat, and how much time I have to check them all lol. It also depends on how much I feed off the springtails, as when there are less I usually feed a little less. I just do a pinch of yeast, yeah not very scientific or specific but it works lol. I would just sprinkle grains over the top, not a ton... just so the grains are sprinkled across the top of the medium. Maybe I will take pictures of how I set mine up, and how I feed them later this week if anyone is interested.