Mastering is a word that gets thrown around a lot these days, more often than not by people who themselves don't even really know what it means. But, fortunately for them, neither do we. That's because mastering belongs in the hands of a proper Mastering Engineer, and we are recording and mix rats who just look that direction of the moon every now and then.
What we do understand though is that there are basically two sides to it:
Aesthetic and Technical.
We concern ourselves with the Sonic Aesthetic side of things, and offer analog stem mastering on the Telefunken–Neumann console. This concept has become widespread now that the world has gone digital - the idea of taking group stems such as vocals, drum group, keyboards/synth pads, guitars, bass etc - and running them through an analog mixing console with master bus compression and equalization at the end, making final prints to digital and tape.
For technically-verified DDP masters, as in - actual master files that are error-verified and set up for replication and distribution, contact a professional facility that knows current standards. CD Baby, for example, offers mastering, replication and distribution services for indie artists on a budget.
If you are going to approach your friend who has a CD robot and wants to be your Replicator, by all means do so, but make sure the files you provide are digitally correct, error-free and Red Book compatible. CD burn systems sometimes have their own ideas about what the right CD format is, and while most computers will read over variations like these, some very high class CD players won't.
For online distribution, streaming services all have their own LUFS targets, and if you go over them, it could be that they will stick your mix up whatever limiter they have, in order to unify loudness across the platform - and in doing so, destroy all the careful nuanced, subtle and balanced mastering work you have done.
It's not rocket science, but by and large the best approach is to do an open and balanced mix with your local studio crazy and hand off the mastering side to a professional mastering artist and then on to a replication and distribution facility. If cash is tight, you can take advantage of a mastering and distro deal, and if you can afford it, hire the Mastering Artist of your dreams and get correct data, and then burn-yer-own with a magnifying glass.