Well that's a big question - the short answer is that modern mastering is a twofold process:
Mastering is Quality Assurance - making sure that there are no technical errors or aspects to the music that would inhibit the listening experience.
Mastering is Sonic Enhancement taking a mix you already love to a new level that allows it to exist comfortably within its respective genre.
The analogy I typically make is that mastering is like varnish on an oil painting - it enhances everything about the work without adding anything new, while also sealing the work against aging.
Here's a quick checklist to make sure:
Is the final mix fully approved? (i.e. everyone involved is stoked about the sound and don't want any more changes)
Are all of the song titles final? (I can help with track order, but I can't name 'em for ya)
Is there any limiting on the master buss solely for loudness? (If so bypass it, if it's part of the sound leave it on)
Are the final mixes rendered in a lossless file format (e.g. WAV) at the native mix session sample rate/bit depth?
Turnaround is generally within a week.
Single master: $75
EP masters: $65/song (up to 4)
Album masters: $55/song
I generally do revisions until the artist is stoked, which is typically within the first or second pass.
If the artist sends over a revised mix after a first mastering pass has been delivered, that new mix is billed separately from the original.
Choosing a Mastering Engineer (ME) is an extremely delicate decision.
You want an ME that:
Won't ruin your music
Has a vibe you connect with so that both the work and working relationship won't suffer
So to truly answer this question I'd encourage you to reach out so I can learn more about the goals and vision you have for your work.
My favorite part of this job is connecting with artists (like you) and helping fellow music makers feel secure when releasing projects into the world.
I have a passion for both sound quality and the artist's creative vision.
The working relationship I strive to maintain is one where the music is as polished as possible while respecting the creative direction of the artist.
No presets. No gimmicks. Custom work for lasting impressions.