I used an old wooden Stampin' Up set of stamps. I've had them for so long that I've forgotten what they're called! I stamped the central "Thank You" circle first and worked my way out to the edges using various shades of Memento ink pads.
Edited to add: This set is called "Seeing Spots" from the 2008/09 Stampin' Up Catalogue - I knew I'd had them a long time, I hadn't realised how long. Sam x