Gather your card making materials including a small plastic cup, Copic Various Ink - blending solution, a paint brush, and a piece of acetate (I use the packaging from different craft products.)
Add the blending solution to your cup and scribble your Copic color of choice on the actetate, You can use a darker color with this method. I've chosen BG75
Dip your brush into the blending solution and then pick up some ink from the acetate. Apply to the background of your image. If it's too dark, add more blending solution. If it's too light, pick up more ink. You can mask your image if you'd like. I use Cryogen paper so I know how the ink moves and I just work the ink close to the image. If you use a paper less coarse, it might bleed into the image so test it first.
The purpose of this method is to get a watercolor-like background in which the color has many shades (using just the one color). Keep working with the ink and the solution until you get a background that is pleasing to your eye. Let it dry (it will lighten with drying)