Make the source files unambiguous
Upload the backing and the vocal as separate files when possible. Name them clearly, then tell the producer which vocal to keep. If an older full-song demo already contains a singer, simply adding a second vocal on top can produce doubling, phase problems or conflicting phrasing.
When replacing a demo vocal, separate the original recording into stems or use an instrumental version. Keep the desired backing, mute the old vocal and use the new performance. The original demo can remain a reference without playing in the final mix.
Timing comes before polish
A clear voice still feels wrong when its phrases arrive in the wrong place. Check the song start, the first downbeat, verse entrances and the last chorus. Silence before a recording begins is not necessarily part of the musical arrangement.
A constant offset and a drifting performance need different treatment. Moving a clip can fix a constant delay. Drift may require phrase-level edits or a more careful tempo map. Avoid forcing a live or rubato performance onto a rigid grid without a musical reason.
One deliverable, with the right parts in it
After the arrangement is correct, balance the vocal against the backing, make restrained tonal changes and export the full song. A solo vocal export is useful for an engineer, but it is not a substitute for the combined result you asked for.
Play the saved export from the beginning and check that the voice, backing, ending and intended duration are all present. The latest-result player keeps that file visible above the conversation. Further timeline edits require a new export; older files remain available for comparison.
Common questions
What if my original file already contains vocals?
Use an instrumental if you have one, or separate the recording first. Adding another vocal without removing or muting the old one usually creates unwanted doubling.
Why is the latest export different from the timeline?
An export is a saved version. Changes made afterwards do not rewrite that file; export again when the new arrangement is ready.