You misunderstand the question, I think. I know HOW to ally myself to a nation in the past, the problem is, I can't get to the past.

I already entered the past once. Before being able to do anything after the initial cutscene, I died. Having not exited any maw from the past, I can no longer get to the past through the maws in the present. I'm hoping I can get to the past through the maw behind the banishing gates in Garliage Citadel, but I don't know exactly if that will work or if there is something else I need to do.