Of course, jala neti is a safe technique provided that the tools used are completely hygienic.
Your neti pot and water must be totally hygienic.
As regards the technique, you need to learn the same from qualified yoga expert. The main thing is to keep your mouth open and breath through mouth (and not nose) while passing the water from nostril so that the water comes out from one nostril to other one. The water must be lukewarm (and not hot) and must be saline. You can measure the tolerance of your nasal to the temperature of the water by putting few drops of water on the back of the palm and if it is bearable, you can use it. Similarly to check the quantity of salt in the neti water, simply taste few drops of the water and check the tolerance of salt. If the taste is ok, you can use the water.
Remember if the water is too hot or cold, you will feel uncomfortable with the neti water. Or if the water is too salty or without salt, again you will feel uncomfortable.
After doing neti from both the nostrils, do not forget to do kapalbhati to remove out all the remnant water in the nostrils. Do forceful exhalations in all directions - up, left and right in order to eliminate all the water. If you do not do such practice, you shall feel headache.
I hope with such instructions, jala neti will be enjoyable to you.
It is very useful technique of shatkarma. After you do it, you will feel fresh all the day.
All the best!.
