I wait for a sunny day and shoot my cards next my southeast-facing window. You want the card to be nearly parallel to the direction the light is coming from, this gives the greatest amount of emphasis on the texture and impression. I use something small like a votive candle holder to prop the card up.
I’ve also found that extreme close-ups, and dramatic angles cropping out much of the card, while perhaps counter-intuitive, are the best way show off the texture.
Examples: http://www.kristinejubeck.com/print/bleeding-hearts-greeting-card/ and http://www.kristinejubeck.com/print/chickadees-greeting-card/
Definitely be sure you have the macro setting turned on, you’ll need it for clear focus on close-up subjects.