Power off the iPhone
Locate the SIM tray on the side of the iPhone
Insert the SIM eject tool or a small paperclip into the tray hole
Press gently until the SIM tray pops out
Remove the SIM tray
Place the SIM card into the tray with the cut corner aligned correctly
Make sure the SIM sits flat in the tray
Slide the tray back into the iPhone
Power on the iPhone
Check for cellular signal and carrier connection