How did William write Macbeth?

We have to speculate about this, because Shakespeare left no diary or other evidence of how he worked. We know that he got his story from Holinshed's Chronicles. He probably read the story of King Macbeth in Holinshed, and thought it had possibilities as a play. He then sat down, in the spare hours he had when not performing, or rehearsing new plays, or meeting with his business partners, or visiting his family and wrote the play out with a quill pen and paper. We don't know how long it took him. We do know that at some point another playwright added some scenes to it, possibly to make it cuter or funnier or more of a musical.