Ion-selective electrodes
Ion selective electrodes are a group of electrodes that are very different from each other. 

The common bits are, a selective membrane, a "sensing" electrode and a reference electrode to measure against. 

The details of the membrane and sensing electrode depend highly on what selective electrode you are trying to build. 

Here is an old patent on a Cl selective electrode for reference:
http://www.google.com/patents/US3740326

Regarding getting accurate results with a DIY: this really depends on which electrode you are trying to build. In the Cl example, I don't see any reason why a DIY would perform worse.