Ron's mother's second cousin would be Ron's third cousin. It goes down each generation, from both sides.

My great-grandfather's first cousin's grandson is my sixth cousin (told ya, I know my sixth cousins). It would go like this:
GGrandpa-MissRuth (his first cousin)
GGrandpa-Sharon (his second cousin, MissRuth's daughter)
GGrandpa-Will (his third cousin, MissRuth's grandson, Sharon's son, my age)
Grandma-Will (GGrandpa's daughter and Will are fourth cousins)
Mom-Will (Mom and Will are fifth cousins)
Me-Will (we're sixth cousins)

that looks even more confusing than it is in real life!!!