From the Brush Selector choose the F-X category and the Fairy Dust variant. Change your brush color to Black. You may want to experiment with the size of the brush. The idea here is to create a frame or vignette around your tree. These masks can be used again and again in your creative process. You may want to experiment with different brushes a well.
When you have finished it's time to save the mask. Name the mask, "mask-1" save in the RIF file format. Select, File > Save As. Close the file completely. You will open it again shortly.


