Step 1
Create a document. Make a new layer and place something on it. I chose this little ball face.
Step 2
Make a new layer above this object and fill it with white. Then press add mask. Is is located near the layers palette(bottom right). Look at the picture for reference to where it is.
Step 3
You should now only have a solid white background with a mask on it. Now grab a round solid brush and set the brush size to 20-30. Make sure your foreground color is black. Begin brushing where the object should be. As you apply a black ink to the mask, it will become transparent and you will be able to see anything on the previous layer.
Final Result:
