I created a random background on a postcard size piece of watercolor paper (4" X 6") I used Pebeo Dyna Acrylic paint that when dry appears shiney or metallic.
The next step after this dried was to place painter's tape all around the outside edge that will later create a frame on your postcard.
I then used a technique that I learned from Paula Phillips (Journal Artista). I painted the entire background over with black acrylic paint. Yes black paint. Do not panic.
Let the black craft paint dry a little but not completely. Remember that craft paint dries quickly.
I placed a stencil - in this case a bird stencil - over the black paint and then took a baby wipe and using a rolling motion with the wipe, remove the black paint inside the stencil. You should be able to see the background coming through.
The final step was to remove the stencil and the painter's tape and let it all dry.
There are a million stencils out there. Have a great time with this technique.
I am also enclosing a couple of practice one I made from random stencils.