The best way is to set the boss/primary target as your focus target and use a focus macro to continue to cs the boss while attacking another target
Thete made a good video if you want a visual guide
https://www.youtube.com/watch?v=DfegJ2bHeEM
This is the best version of the cs focus macro (below) imo, resolves some of the problems with the simpler ones I see around eg: will cs your normal target if you forget to set a focus target, keeps working if the mob on focus dies or if you focus a friendly target to do something else.. etc
Code:
/cast [@focus,exists,nodead,harm] [] Crusader Strike