How do I register my SmarTrip card?

This information comes from WMATA.com.

The most efficient way to register your card is to add it to your WMATA online account. You can also fax a note with the SmarTrip® card serial number, your name, address, and email address to 202-962-1035 or mail the same information to:

SmarTrip Regional Customer Service Center
PO Box 220568
Chantilly, VA 20153

Note that if we mail a new card to you (e.g., to replace a lost card), we automatically register the card on your behalf. However, if you have an online account, we recommend that you also add the card to your account.

You will find pictures of the various card types on the WMATA website.