Learning via a specific course (Udemy, or a paid course from somewhere like Smashing mag).
Doing a more formal university course or degree.
Working very closely with a talented designers to absorb some of their expert knowledge.
Lots of practice from initially copying existing designs and understanding why decisions were made and gradually building up your own designs from there.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
Good question, a combination of a few things: