First make a selection of the area you would like to mask. Using your selection tool of choice for this post I will be using the Quick Selection tool.
Using this tool I quickly select the image of the bird.
I then open the mask tab and click on “Select Pixel Mask”
If you selected the background instead of the object to mask you many need to hit the invert button.
You now have a layer mask.
To add to or take away from that mask click on the layer and select the mask.
Using your paint brush tool you can use black to Mask more or White to remove the mask.