You look at topographical maps, go by watersheds, the evolutional of territories and districts, the administrative divisions used by the non-Americans when dealing with a land, and then you make changes to make it more like IOTL due to it being a good idea. Such as how the borders of Mexican Baja and Alta California are the same before the Mexican-American War as they are IOTL again, how Idado gave up lands to Montana and Wyoming because they were on the otherside of the Continental Divide, how the IOTL border of Utah is in part due to how their are mountains seperating what would have been the northeast corner if they made the state a rectangle, and... I really don't know what to say about Victoria.
I would say look over this at random. You will be bound to find some interesting territorial boundaries that happened back before anyone really knew what the lay of the land was like. So many fun borders were created due to people being mistaken on how far something was. And while I am unsure if it was true, there may have been problems with US-British borders in part due to the crease of an atlas. Might be thinking of another example, though.
https://en.m.wikipedia.org/wiki/Territorial_evolution_of_the_United_States