1) We went to this link and filled out the proper form that fit our criteria, which is explained on this link, http://travel.state.gov/passport/ppt_card/ppt_card_3926.html
2) After completing the proper form from Step 1, find your nearest post office and determine if they do passport cards by calling them. Here is a link to where you can find your nearest Post Office http://usps.whitepages.com/post_office
3) If you are not eligible to get a passport card via mail, which you will learn in step 1, then schedule an appointment to go to your post office and turn in the necessary documents.
4) Get passport pictures and gather all documents required which is explained on the forms you complete from step 1. You will need these these documents at your appointment.
5) Go to your appointment and then once complete, you will receive your passport cards in the mail in a few weeks.
We had to make an appointment because we were getting passport cards for children. The appointment took 5-10 minutes, we left and received the passport cards in the mail in three weeks.