In cells, there are two half reactions. Firstly you have to work out what the cathode and anode reactions are
Using a data sheet, look at the list of standard potentials and find the half reactions you need
You will notice that all the half reactions you see are reduction (gain of electrons, occurring at anode)
For you oxidation half, you need to switch the voltage sign from positive to negative or vice-versa.
SO get the two voltage values and add them.
Then you will have the overall cell potential
this applies for both lead acid and button cells