Understand how to become popular (5,2)
I believe the answer is:
catch on
I believe this is a double definition.
'understand' is the first definition.
(thesaurus)
'to become popular' is the second definition.
('catch on' can be a synonym of 'become popular')
'how' is the link.
(Other definitions for catch on that I've seen before include "twig" , "Understand - become fashionable" , "Get it" , "Informally, become popular or come to understand" .)