1. Optionally create a rough master. Just like a rough mix, attempt to master the track(s) yourself, and include them with your final mixes. This gives me an idea of what you are looking for in your final master’s. If you don't want to do a rough master, skip to step 2.
2. In your DAW session, take any limiter or compressor plugins off the mix bus.
3. Make sure your track does not peak above -6 DB on the mix bus. This is an arbitrary number but this ensures there is no clipping in your mixes, I cannot fix a mix that is already clipping.
4. For stereo masters export your track as a mixed down stereo 24-bit WAV file. For stem masters, export your track as 2 to 6 logical stems, i.e. all your drum tracks as a stem, all your guitar tracks as another, etc. These should also be 24-bit WAV files, I do not accept MP3s.