Dear Mukund, You need to practice jala neti first time under guidance of qualified yoga instructor. However, I may outline the precautions that need to be taken while any shat karma, i.e. yogic cleansing actions:
(i) Any shatkarmas including jala neti should be practised on empty stomach.
(ii) Jala Neti is advisable in the morning.
(iii) The Jala neti water should be luke warm and salty. How much? That you can measure by putting few drops of jala neti water on the back of the palm, if it is comfortable, use it. Similarly, you can taste the water, if it is comfortable, use that much amount of salt only.
(iv) During jala neti, when you insert nozzle of neti pot, keep ensured that your mouth is opened and you breath through mouth and not nostrils; only then the water will flow smoothly from one nostril (to which nozzle is inserted) to other nostril out. If you breath through nostril during jala neti, it will harm you.
(v) After you are done with jala neti, do kapalbhati to dry out your nostrils that are irrigated with water. Any water remained in the nostril after jala neti may cause headache or discomfort for the whole day. To check whether your nostrils are dry enough, just exhale forcefully over the back of your palm, check if the exhaled air contains the water droplets, if not, stop kapalbhati.
I hope that I have been able to satisfactorily answer your question, if you find other clarifications, please feel free to write here.