What is the best workflow for improving a song?

For most songs, use this order:
  1. Split the song into stems if you need to fix individual parts.
  2. Restore or enhance the stems that need work.
  3. Use EQ, volume, or Match EQ to balance the stems together.
  4. Export the finished stem mix.
  5. Import the exported mix again and use Master as the final loudness step.

In other words: restore and mix first, then master the final mix.