H3PO4 + Ca(OH)2
First lets create a balanced chemical equation for these reactants. Phosphoric acid donates 3 protons and calcium hydroxide donates 2 hydroxide anions. So add coefficients to make a total of 6 of both species.
2H3PO4 + 3Ca(OH)2 --> 6H2O + Ca3(PO4)2
The ionic equation is written with all of the ions as separate species. Calcium phosphate is soluble in water so it too will appear as ions.
6H+ + 2(PO4)3- + 3Ca2+ + 6OH- --> 6H2O + 3Ca2+ + 2(PO4)3-
The net ionic equation eliminates the spectator ions, or the ions that do not participate in the reaction. This eliminates the calcium and phosphate ions.
6H+ + 6OH- --> 6H2O