There is no quick way to make a custom tech tree. I would start by looking at an existing tech tree that is at least reasonably similar to what you want to do, then save it as a new tech tree (with a new name) using the tech tree editor.
You can rename technologies there without ill effect, because each tech has an internal name which is what is used by the game when deciding what pre-requisite is needed to research a tech.
As I said, when you edit a specific race's TechTree, any changes you make there will be recorded only in their tech tree. Any information which isn't in there is looked up on the main TechTree.