As a regular Nissan driver, stand near the car holding the key with all the doors shut and press the button on the door handle. That's all. Or press the lock button on the remote.
It won't let you lock your keys in the car so:
If there are keys, spare key or main key inside the car then it will refuse to lock and say "cheep cheep cheep". So maybe keep the key a foot or more away from the car door.
The only problem I've ever had with Nissan keyless is that every now and then the car insists that the key is placed on the start button to sync up. System works really well in my experience.