You can mail in the IRS application form to obtain an ITIN, Form W-7, along with required documentation substantiating foreign/alien status and the applicant's true identity from outside of the U.S.
Alternatively, Form W-7 with required documentation can be presented in person at IRS walk-in offices, or processed through an Acceptance Agent authorized by the IRS.
To obtain an ITIN, you must complete IRS Form W-7, IRS Application for Individual Taxpayer Identification Number.
The Form W-7 requires documentation substantiating foreign/alien status and true identity for each individual. You may either mail the documentation, along with the Form W-7, to the address shown in the Form W-7 Instructions, present it at IRS walk-in offices, or process your application through an Acceptance Agent authorized by the IRS.
The IRS provides the following instructions on how to an apply for an ITIN:
How
do I apply for an ITIN?
Use
the latest revision of Form W-7, Application for IRS Individual Taxpayer Identification
Number to
apply. Attach a valid federal income tax return, unless you qualify for an
exception, and include your original proof of identity or copies certified by
issuing agency and foreign status documents.
Because you are filing your tax return as an attachment to your ITIN
application, you should not mail your return to the address listed in the Form
1040, 1040A or 1040EZ instructions. Instead, send your return, Form W-7 and
proof of identity and foreign status documents to:
Internal Revenue Service
Austin Service Center
ITIN Operation
P.O. Box 149342
Austin, TX 78714-9342
You may also apply using the services of an IRS-authorized Acceptance Agent or visit some
key IRS Taxpayer
Assistance Center in lieu of mailing your information to the IRS in Austin.
Taxpayer Assistance Centers (TACs) in the United States provide in-person
help with ITIN applications on an appointment basis only. The IRS's ITIN Unit
in Austin issues all numbers through the mail.
For additional information about ITINs, please see the following IRS topic:
https://www.irs.gov/individuals/general-itin-information