An engineer can only change what arrives as separate parts. Almost every slow, frustrating mix starts with a session that did not.

Stems, not a bounce

Stems are the song exported as separate files, one per instrument or group: vocal, backing vocals, drums, bass, guitars, keys, and so on. A stereo bounce is all of that flattened into two channels.

A bounce can be mastered. It cannot be mixed, because there is nothing left to move.

All starting at the same point

Every file from the same timeline position, usually bar one, even the ones that do not come in until the second chorus. The silence at the start is what keeps them lined up.

Stems trimmed to where each part happens to begin is the single most common way a session arrives unusable, and reassembling it by ear is slow work that nobody wants to pay for.

Name them so someone else can read them

Audio 7_01.wav tells an engineer nothing. Vox_Lead, Gtr_Acoustic_L, Keys_Pad tells them everything.

If two files are a stereo pair, say so in the name. If a part is a double, say that too.

Leave some headroom, and take the master bus off

Export with a few decibels of room at the top and nothing on the master bus. No limiter, no bus compressor, no “mix glue” on the way out.

Processing that belongs to the sound can stay: a guitar amp, a deliberate vocal effect, a synth you printed with its character. Say which those are. The distinction worth drawing is between processing that was a creative decision and processing that was an attempt to fix a balance problem, because the second kind is exactly what a mix is for and it now cannot be undone.

Send your rough mix

Whatever you were listening to while you made it. It is the clearest statement of intent you can send, and it costs you nothing.

An engineer who has heard your rough knows which parts you care about. One who has not is guessing, and the first version will reflect it.

Say what the record is for

A song going to streaming and the same song sitting under a film are mixed differently, because in the second one dialogue has to survive on top of it.

Knowing the destination before the work starts avoids a mix that is technically fine and useless for its actual purpose. It is one sentence, and it is the one most often left out.