larger cations occupy the position of $F^{-}$ ions and smaller anions that of ${\text{C}}^{+2}$ ions 

smaller cations occupy the position of $F^{-}$ ions and larger anions that of ${\text{C}}^{+2}$ ions

${\text{X}}^{-2}\text{ions occupy all the 8 octahedral voids}$

each ${\text{X}}^{-2}$ is surrounded by ${\text{4 M}}^{+}$ in tetrahedral arrangements