Oh it’s quite simple! If you have photoshop, right click on your layer>blending option> drop shadow. That way when I use my white brush a shadow follows on the bottom, which creates a pop out effect. You can also alter the angle of the shadow in PS. You can also do it manually by outlining the desired object with a white brush and then create a layer underneath, set to multiply mode, and use a soft brush to outline the white to manually create a shadow. Did that make sense? Or should I make a mini tutorial? x_x