Fund and arm a rebellion in a small sliver of a neighboring state.
Drive millions of people out of that area.
Invade driving more people out of the area and killing many.
Hold a vote by the remaining people which you didn't kill, organized and counted by the military, and throw in a couple of million people no longer residing in the area because you forcibly relocated them to your territory. But don't count the people who relocated to the other side. And do all of that with no oversight.
Annex the area in violation of all international norms and laws and the UN charter that you signed.