Start with a purpose
Extract stems to solve a named problem: vocal masking, low-end conflict, excessive cymbal energy, weak transition or arrangement edit. Separation alone does not improve the song.
Inspect separation artifacts
Listen in context and solo. Phasey tails, leaked drums, watery vocals and missing transients may become more obvious after processing. Use the cleanest path that preserves the performance.
Rebuild relationships
Balance lead, support, drums, bass and music around the song's priority. Use subtle ducking only where elements compete: kick against bass, lead against choir, or response vocals against the instrumental bed.
Print a stable premaster
Leave headroom, avoid clipping and compare the rebuilt mix with the original at matched level. Export at the project sample rate before mastering.
A stem is useful when it creates a better decision, not merely another file.