Check that the phone is eligible for unlocking
Make sure the device is paid off
Confirm the account is in good standing
Use the original T-Mobile SIM or account if required
Request the unlock through the T-Mobile app or website
Contact T-Mobile customer support if the unlock option is not available
Follow the unlock instructions sent by T-Mobile
Insert a non-T-Mobile SIM to verify the phone is unlocked
Restart the phone if needed
Reset network settings if the new SIM does not work