Make selections with the Magnetic Lasso tool

Select the Magnetic Lasso tool  (L) from the toolbar. If you can't find the Magnetic Lasso tool, click and hold the Lasso tool to show the other related tools, and then select the Magnetic Lasso tool.

In the tool options bar, specify a selection option: New Selection, Add to Selection, Subtract from Selection, or Intersect with Selection.

In the tool options bar, choose to turn on feathering and anti-aliasing. Also, set any of these options - Width, Contrast, Frequency, and Stylus Pressure.

Click anywhere in your image to set the first fastening point. Move the pointer along the edge you want to trace. As you move the pointer, the active segment snaps to the strongest edge in the image.

Click once to add a fastening point manually. Continue to trace the edge of your image and add fastening points as needed.