Access Form DS-82 on the U.S. Department of State travel website. Click on "Passports" then on the "U.S. Passport Card" tab. Under "How To Apply" there is a link to the form you need. You can complete this online.
Fill out the required information according to the instructions on the form.
Print the completed form. Use black ink and single-sided pages.
Sign and date the first page of the application.
Put the signed form, your passport, two passport photos and payment into a padded envelope. As of April 2010, the cost of a passport card by mail is $20. The passport card has the same period of validity as the passport book: 10 years for an adult and five years for minors under age 16.
Mail this envelope to the address listed on Form DS-82. You should get your card in four to six weeks, according to the State Department website. Expedited service is available.