Skill building requires skills identification. This first steps goes through several 1 to 1 meetings to identify where a designer stands on the wide UX/UI design scope. Using visual references, Figjam files, and having an open and trust-based conversation help bringing a clear view of the skills level. 

I believe that skills come though practice. Once skills levels are identified and objectives are set (in regard of the designer's wishes and the projects or products requirements), I look for tasks and missions that would hit the spot. During their realization, I give feedbacks and provide examples and bests practices. 

After a year, we go through skill identification again and observe the difference.