Hi Courtney,
I can find varying directions for potting calathea ranging from 2/3 indoor potting mix and 1/3 peat, to no peat indoor potting mix. The key item all sources agree upon is that calathea needs well draining soil but they like humidity. Hence it is suggested to place the pot on a pebble filled saucer or tray which is kept water filled to increase the humidity. Apparently the leaves will scorch if there is a dry draft, i.e. fan, or dry soil or a soil high in salt from over fertilizing. These are understory plants which are native to the jungles of the Amazon so try to duplicate those growing conditions--warm, moist, well-drained, low light.