If the contrast between hair and the background is strong, cutting out hair is easy. Just use ‘Quick selection tool’ and ‘Select and Mask’ like this page.
But the typical image between hair and background is more vague.
How about this image?
This old woman’s right side hair is difficult to see. If you try cut out her hair by ‘Quick Selection tool’, you will get the image like below.
Look at the image above. The right side hair is apparently bad. So what should we do?
One way is using other parts of hair instead. In this case, use the left side of her hair that is more clear for the right side.
I explain step by step.
At first, create the selection of her hair by clicking the layer mask with ‘Ctrl’ .
Go to ‘Select’ and choose ‘Transform Selection’ 
Now you got the selection of her hair. And you can transform this selection freely. For example, choose ‘Flip Horizontal’.
You can move the selection. Then choose ‘White brush’, and ‘Pint’ inside the selection.If you mimic this process several times, you will get better image like below.
