To initiate a formula, type = followed by the function name if appropriate, and the individual cells you want the formula to act upon.
For instance, If cell A1 is empty, and I want to subtract cell C1 from cell B1, I would enter in cell A1:
= $B1 - $C1 and click the green check mark to set the equation in cell A1. If you want to edit the equation, double-click in cell A1. Once you have entered or wish to edit the equation, a list of functions will appear in the right Pages v5.2 panel.
For non-contiguous cells, say B3 and C3 have values that we want to add to the previous equation, with results written to A1:
= $B1 - $C1 + ($B3 + $C3)
And if I wanted to know if the cell A1 is divisible by 2,
= MOD($B1 - $C1 + ($B3 + $C3, 2) will return 0 if true and a remainder of 1 if not.