How did they turn aether into the reality stone?
I think reality stone due to it's nature can do many things and take many different forms. I think Malekith, the leader of the Dark Elves turned reality stone into aether, a liquid form and a weapon he wanted.
To turn aether back into reality stone I think you first need to be able to control it to manipulate it's reality. Someone like Thanos absolutely can and turned it back into a red stone to fit it onto his gauntlet.
We knew Thor or Asgardians could not do it, Thor couldn't even touch power stone. If Asgardians were able to, I image they would have turned aether into another form with a different purpose.
So I image it was Hulk who turned aether into a red gem in this movie, he is the only avenger can hold (direct contact) an infinity stone (time stone from ancient one) without any problems, but I can't imagine how he would have acquired the knowledge about this. Dr strange might know it, but he was dead and we didn't see avengers time traveled to consult him. Ancient One might know it too but I didn't see Banner asked about it.
I don't believe the movie writers have thought about this at all. I only watched the movies and read some wiki pages, but never read the comic books, so anyone else know more about this or have a different theory?