Watch these videos for more practice on how to calculate formal charges. NOTE: In these videos, the term "bonding electrons/2" is the same as "# of bonds" since each bond represents two electrons. "Nonbonding valence electrons" are the same as "lone electrons."
Source: Formal Charges: Calculating Formal Charge, Wayne Breslyn, YouTube
Source: Calculating NH3 Formal Charges, Wayne Breslyn, YouTube
Source: Calculating CO2 Formal Charges, Wayne Breslyn, YouTube
Source: POCl3 Lewis Structure, Wayne Breslyn, YouTube