Power 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 SIM tray
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
Push the tray in until it is fully closed
Turn the iPhone back on
Check for cellular signal and carrier connection