I agree with alucard_my. Routine is what you must do. Don't get lazy if you know what I'm mean.
For my case, my baby girl knows to say "Ng Ng" or "Shi Shi" when she was 1 year old.
Never allow her to wear pampers/ diapers, only napkins.... yes! you may have to wash it every day or every time they missed it and wet themselves.
Then make it a routine to have them sit on the potty every half hours. Increase it when they able to hold longer.
At first, they may only finger point the potty. Never mind, let them sit on it. Once their business is done, told them it is "shi shi" or "Ng Ng". Slowly they will know.
Of cause, if they we themselves. You may need to give an angry face and small punishment to them. So that they know to give signal and you are angry if they did it on purpose.