Separate the half reactions, look up the standard reduction potential and calculate potential different

eg if the cells are standard and you have the reaction 2Ni2+ + Mg ---> Mg2+ + Ni. The half reactions are

Mg---> Mg2+ + 2e- (has -Eo of +2.37V = this is a -Eo as it has been reversed compared to the reaction in the standard reduction potential series ) and Ni2+ + 2e- ---> Ni (Eo of -0.23V. So the cell potential would be 2.37 + -0.23 = 2.14V. The values are added because the two equations are added to get the overall reaction

Separate the half reactions, look up the standard reduction potential and calculate potential different

eg if the cells are standard and you have the reaction 2Ni2+ + Mg ---> Mg2+ + Ni. The half reactions are

Mg---> Mg2+ + 2e- (has -Eo of +2.37V = this is a -Eo as it has been reversed compared to the reaction in the standard reduction potential series ) and Ni2+ + 2e- ---> Ni (Eo of -0.23V. So the cell potential would be 2.37 + -0.23 = 2.14V. The values are added because the two equations are added to get the overall reaction

Mg---> Mg2+ + 2e- s -Eo =2.37V

Ni2+ + 2e- ---> Ni Eo =-0.23V

Mg + Ni2+ ---> Mg2+ + Ni Cell potential = 2.37 + (-0.23) = +2.14V