Turn off the iPhone.
Locate the SIM tray on the side of the iPhone.
Insert a SIM eject tool or a paperclip into the small hole next to the tray.
Press gently until the tray pops out.
Remove the tray from the iPhone.
Place the SIM card into the tray with the notched corner aligned correctly.
Make sure the SIM card sits flat in the tray.
Slide the tray back into the iPhone.
Turn the iPhone back on.