about a year ago I had 3 leaking fuel injectors. i took it to the shop and they replaced them with new ones and charged me about $900. now the last one has sprung a leak so I bought an o-ring kit. can anyone tell me how to put them in it will be my first time under the fuel rail. any help is appreciated

oops I feel stupid....I have a 1989 240sx w/ ka24e and about 155k miles

Well if the injector is leaking fuel chances are the injector itself is bad and you have to replace it. The o-rings you asked about seal the injector to the rail and to the int.manifold. Clean the injector with brake cleaner and start the car and see where it leaks. Or just put another one in and replace the o-rings at the same time. 900 bucks man I would like some of that I could replace yours and mine and have money for beer.