How do calculate formal charge in a molecule? In detail please.

In case of of the polyatomic ions , the net charge is possessed by an ion as a whole and not by a partcular atom. However for some purposes each atom in a polyatomic molecule/ion is assigned a formal charge. It is basically the difference betwwen the number of valence elctrons in an isolated atom and the no. of electrons assigned to that atom in a Lewis Structure. 

Formal Charge = Total no of valence electrons in free atom – total no. of non bonding (lone pair) electrons 

                          -1/2 total no of bonding (shared ) electrons.

The most stable structure of a molecule is thee one which has smallest formal charges on the atoms.

For example for CS₂ ie S=C=S

F.C. on each S atom = valence e^- – lone pair e^- -1/2 bonding e^- = 6 - 4 -1/2 X4     =0

F.C. on C atom = valence e^- – lone pair e^- -1/2 bonding e^-     = 4-0-1/2 X8 =0