Before hiring an online mixing service, the most important thing is to properly prepare your files. A poorly exported stem can ruin the engineer's work, no matter how good their plugins are.
Step 1 — Bypass everything on your Master channel: reverbs, compressors, limiters, EQs. Export clean. The mixing engineer will apply their own processing chain.
Step 2 — WAV format, 24-bit, 44.1 kHz: never export as MP3. Lossy compression destroys the transients the engineer needs to work with.
Step 3 — All stems starting from 0:00: even if a track only comes in at the second verse, it must start from the beginning. This keeps everything in perfect sync when importing.
Step 4 — Name your stems clearly: use names like "lead_vocal", "808_bass", "kick", "synth_melody". Avoid names like "track01" or "audio3".
Step 5 — Compress everything into a .ZIP and share via WeTransfer or Google Drive. Never via WhatsApp or email, which automatically compress audio.