Unscrew the stock metal Nissan antenna at the hexagon base. You can probably do this by hand, but a wrench can help.
Remove the antenna screw adapters from the replacement antenna kit (should be 3). All three have the same thread pattern on one side, and different thread patterns on the other side for adapting to your vehicles specific thread. Try each adapter thread until you find one that matches. Screw in the correct adapter screw.
Take your new flexible antenna and thread it onto the new thread extending from your vehicles antenna base.
That's it.
I bought mine in canada and had a problem with the threads coming off inside the antenna (the threads weren't long enough). I ended up picking some small o-rings and placed them between the Nissan base and my new antenna. This kept the threads in correct position to hold the antenna.
I think it might just be a Canadian thing. I haven't heard any US purchasers having the same problem I did.