If you focus on a foreground subject, using a large aperture of f/2.8 on your lens, and then place a string of Christmas lights some distance behind it this will result in the points of light being out of focus and blurred; they become large, diffused orbs. The effect is quite lovely, but it doesn’t end there. With a little ingenuity, you can turn those orbs of light into specific shapes.
Make a black paper cylinder that wraps snugly around the end of your lens and tape it together, so it is secure. The cylinder only needs to be about 30mm-50mm deep. You can then set it on another sheet of paper, trace around the circumference of that tube and cut out a disc that covers one end of the cylinder.
Use a sharp craft knife or scalpel to cut a custom shape from the centre of the disc. A heart, a star, a snowflake, whatever suits your needs. The shape you cut can be about 25mm wide. The disc can then be taped to the cylinder, which is placed over your lens.
Use a large aperture on your lens for shallow depth of field and focus on a close foreground with your background lights a distance away. When you take the shot, those out of focus orbs of light should now take on the shape of whatever is currently cut out of the disc taped to the front of your camera.
You can experiment with a range of shapes and try each of them on the front of your lens.