Create mask with brush

On the Lightroom on the web homepage, select All Photos.

Select an image to open it in detail view, and then select Masking .

Within the Masks panel, select Create new mask. 

Select Brush.

Adjust the sliders for the following options:

• Size: Specifies the diameter of the brush tip in pixels.
• Feather: Creates a soft-edged transition between the brushed area and the surrounding pixels. The distance between the inner and outer circle of the brush represents the feather value.
• Flow: Controls the rate of application of the adjustment.
• Density: Controls the amount of transparency in the stroke.

Select and drag the Brush tool over the areas you want to edit.

Control the overall selected area of the Brush mask using the Amount slider.