Question- Editing

Hi professor Jerry-

I was wondering if you can give us some tips when using "lasso" then selecting to "Brightness and Contrast" and when I hit okay, It shows a line showing what I just did. I tried to be as precise as possible to highlight the area but It's impossible to get it perfectly. Please take a look at the pic to see what i'm talking about. The lasso is between the mountain and sky. How do I get it to be smooth? Clear? faded....