The process of forming a Theory of Change is about coming up with a map of how change will come about in the world. It starts by a group building a shared vision of it's particular desired end-state is.You then proceed to build a graph of dependent states back from that end point to the current state of the world. This is called 'back-casting'.
The process has several steps including things like surfacing and testing assumptions, detailing how you would measure successful change at each step, and assessing the cost and nature of intervention to prgress between states.