It's hard to mess up watercolor. If the final result doesn’t look great, don’t sweat it. Sometimes the ugly ones look cooler when you cut shapes from it.
If you don't have stencils, make your own or trace around household objects such as a coffee mug or toilet paper roll.
If you're using a wet glue, like tacky glue, use it sparingly. A lot of glue might cause the card to curl or dimple.