Start with the files you actually have.
Before you request a mix, open your delivery folder and describe its contents. One stereo instrumental, separate instruments and grouped exports are three different starting points. Do not assume that a file named ‘stems’ contains every instrument on its own.
Send the engineer a short inventory: one instrumental WAV, one lead vocal, two doubles and one ad-lib track, for example. That description is more useful than ‘everything is included.’ Keep an untouched copy of the original delivery before making changes.
Describe the level of separation.
A stereo beat combines the instrumental into one audio file. Individual trackouts separate musical parts, such as a kick, bass or piano. Grouped stems combine selected parts, such as all drums together. Sellers and collaborators may use these labels differently, so ask for the actual file list.
A practical question is: ‘Can the engineer change the bass independently of the piano with these files?’ The answer tells you more about the available control than the package name.
Agree on the handoff before exporting.
Ask whether the engineer wants individual tracks, grouped stems or your existing stereo beat plus vocals. Confirm the common start point, export length, file format and treatment of creative effects. Do not automatically remove a sound-defining effect or combine separate takes.
For a software example, Ableton documents separate-track export and equal-length files in its audio export manual. Follow the instructions for your own software.
Keep files and permissions together.
Keep the applicable agreement with the exact beat title and version. Ask the seller about missing trackouts instead of assuming a different package is included.
Follow the preparation checklist, then submit a mixing request. The current private request workflow accepts up to 12 audio files, with a 64 MiB limit per file. Ask about an agreed transfer method for larger projects; do not merge tracks simply to fit the upload count.