Despite the many Nissan models, one set of instructions programs the smart key for almost every model and model year. Entering the programming mode deletes the car computer's memory of all prior authorisations, so you'll have to program all your remotes as a set in a single session.
Close all doors and lock them using the driver's door control panel.
Insert the ignition key, but don't turn it. Take it back out.
Repeat Step 2 five more times, all in under 10 seconds. Don't do it too fast, though. If you did it to the right count and right speed, the car will flash its hazard lights.
Push the lock button on the first remote within five seconds of ending Step 4. The hazard lights flash again if you did it right.
Unlock and relock all the doors from the driver's door control panel, if you have another remote to program.
Push the lock button on the next remote within five seconds of completing Step 6.
Repeat Steps 6 and 7 for each additional remote.
Remove the ignition key.
Unlock the doors from the driver's door control panel.
Open the driver's door. This completes the exit from the programming mode.
If the above directions don't work for you, look for your Nissan's model and model year at the link below. Some recent years don't allow manual reprogramming, forcing you to go to the dealer, who may charge up to £65 per key. Earlier models don't require you to press specifically the lock button on the remote--just any key. However, for simplicity it's best to just press the lock button since that works for both situations.