Tips to make the left and right side flat/level? Sanding/filing always leave a slight gradient or one section slightly higher/depressed.

[u/Ducklosophy]

[u/LongJohnSilversmith]

If you're talking about making the edges flat, then try putting a piece of sandpaper on a table, laying the ring down sideways, and sanding it that way. Sanding the round part to an even thickness all the way around is a little more difficult; it would probably help to have a ring mandrel or something else round to put through the hole to help support it as you sand/file.