Some of the techniques I use elsewhere are creeping into my ATC's. Stitching on card is something to do while watching TV or listening to the radio. I usually prick 3 or 4 layers together during the day when the light is good, then select a graphic to go with the embroidery, and choose thread colours to enhance the graphic. As thread for a card does not have to withstand washing, it is a simple matter to space dye thread with felt tip pens or watercolours.
Here are examples of cards
and an ATC.