DAB quantification of leave

Hi everyone. I am new to Image-J and have no pro-founding knowledge on image analysis.
As i looked through this forum, people did their DAB staining on tissue level. For me, I would like to quantify the stained area of a leave (i.e. percentage of stained area).

Below is one of the sample images.

Can anyone please help me with this? If possible, i would like to learn about ‘create mask’ and ‘thresholding’, as i looked through the internet, i was required to do both of this.
Thank you in advance.