The engine in a G35 uses a direct ignition system,,in other words it is coil on plug.
Each spark plug has its own coil and the coils are arranged in a row on each cylinder head bank.
You need to remove each coil/ coil pack,,then you can remove each spark plug.
Do one cylinder at a time,,you will need to use a deep reaching spark plug socket to remove each plug,,get the kind with the little rubber boot inside of the socket,,it will grip the plug,,that way the plug won't fall off when you are removing the socket.
When you get ready to screw the plug back in,,put a little bit of anti-seize compound on the threads of each plug- Napa # 765-1674 or equivalent.
Then before you put spark plug boot/ coil pack on the plug ,,put a little dielectric grease on the inside of the boot,,this will prevent corrosion and will aid in a good electrical conduction,,you can use G.E. # G 635 or equivalent