1. First choose a light background color (I used D0F2F0) then use the same color and darken it for the foreground color (I used 299691).
2. Open a new image, 400x400 and add a new layer using the default values.
3. Click on your brush tool, and select your custom brush.
4. Apply the brush and click image/effects/cutout.
The settings are Fill interior with color checked, interior color/foreground, shadow/white, opacity/100, blur/6, vertical and horizontal/3. Apply the effect.
6. Click image/effects/drop shadow.
The settings are black, opacity/100 blur/9.4 vertical and horizontal/2
7. merge your layers and crop to the desired size.
When I made the brushes, I just chose those settings at random and really liked what I got. You can experiment with different settings, or apply a texture to your background first for an even nicer effect. Have fun!
I have added more brushes in this series. They are all corner brushes.