To make a cut-out, paint masks over your image to indicate which parts to keep and which part to remove. Fluid Mask uses three types of mask:

To create a mask, you simply paint the image with the brush tools located on the Tool Palette.
Paint keep
masks using the green-colored tools.
Paint delete
masks with the red colored tools.
Blend
masks are normally applied automatically, but you can paint them yourself
using the blue-colored tools.