The novel only traces the criminal career of Sonny. We read that he commits a clumsy armed robbery as a kid, then Clemenza takes him in hand and teaches him the gun, the knife, the garrot, etc.
SPOILER!
Later the Don is wounded by an Irish gunman, and Sonny steps in, showing a "genius for urban guerilla warfare." Presumably that's how he made his bones.
Nothing is said about any violence connected to Tom and Fredo. No doubt Tom "made his bones" when he graduated from law school.
Why not buy or borrow the book and read it? You'll be glad you did!