The best way I can describe them is they are like the gateways from stellaris. You just have to right click on the portal's location on the overworld, and your armies will walk there. Each portal in the nether goes to a different portal in the overworld, though, and it's hard to tell where the portals are in the nether once you conquer all of it.

suggest making a Portal, or a symbol on the land to show them

im trying to go through as the southern empire, how do?