For candle pots, or pots that will not be used for plants, use a water-based polyurethane or craft acrylic sealant. Patio Paints are a good brand for terra cotta. You can find Patio Paint at most craft stores. If you purchase Plaid Decorator Blocks to decorate your pot, they come with a pattern suggestion sheet and how-to instructions. You can also buy more than one set of blocks and mix patterns and shapes.
Plaid does offer their products for sale at their web site.
See below for specific suggestions. To decoupage your flower pot, pick any wrapping paper, wall paper, greeting card, or fabric, etc., that you like and make a color copy of it. Then seal the outside with a water based polyurethane 2-3 coats. There are several of these on the market, available at craft, hardware and paint stores. Check to see if the product you select is OK to use over a painted surface.