How to run procedure when player crouches & right-clicks sign?

I am trying to make a mod to get more customizable signs where my first item is a "bolding agent" which when crouching & right-clicking on a sign, it turns the text on the sign bold. Problem is, I have no idea how to do that. I just mostly need help with the part to run a procedure when the player right-clicks a sign while crouching with the item.

Thanks in advance!