Form W-7, supporting documents and the verification options to discuss with an advisor.
An ITIN is applied for with Form W-7 alongside proof of identity and foreign status.
Sending an original passport by post is not the only route: certified copies from the issuing agency, or an IRS-authorised Certifying Acceptance Agent, can verify documents for you.
Which option fits depends on your reason for applying, so it is worth discussing before you submit.
This is a short summary for general information. Rules change and every situation is different, so speak to an advisor before acting. See our USA services.