An engineer can only mix what you send. Most delays on a record are not the mix; they are a second and third round of “can you resend the…”. This is everything they need in one delivery.
Before you export
5 steps
What to export
5 steps
How to send it
4 steps
Reference tracks
Two or three songs whose vocal sound you want, and one line each on why: “the vocal this upfront”, “this much space on the hook”. It is the fastest way to skip a revision.
What not to send
Session files from your DAW unless asked, MP3s of the vocals, a beat with tags still on it, or a 3 GB folder of every take. The engineer needs your chosen takes, clean, lined up, and named.