I want to start the work of mixing and mastering but I am totally new. I want to know how the whole process of communication happen with clients. I have heard that beginners are supposed to do quite some free works to build rapport. My doubt is why would they give their tracks to such amateurs, don't they fear their music may get stolen? How is trust established for online and amateurs? Are there some copyright and contract based protocols that I need to learn for the mixing and other related jobs? If you can provide link or YouTube channels on these issues, it would be great.
Although I don't cover this topic on my YouTube channel, I have done quite some contracting work for clients. My experience is: there is no recipe for building rapport. But there are multiple possible approaches. Here are a few thoughts that are probably far from complete or applicable in all cases.
I would start with projects on your level. Don't expect to mix big artists when you're just getting started. Do great work for smaller artists and you will build rapport.
Also be polite and reliable in everything else you do. This pay people will trust you sooner.
Never ever break a promise. If you promised not to use a certain mix for your own marketing, stick to your promise. Everything else will wipe out years of built rapport.
Another tip: do free work, but do not overdo it! Your work has a price and people judge your quality in part by the price you call. Get comfortable with clients turning you down for that – in the long run it will strengthen your rapport.
And for bigger artists: they might give out their tracks to several people like you in parallel and simply take (and pay for) the version they like best. Kind of a contest. Delivering good work here can be a way to elevate your circle of clients. Even if they don't pick your mix, they now know your style, that you deliver quality and that you're reliable.
As for the fear of music getting stolen: that is a very real thing for big und upcoming artists. Some have been ruined by leaked album releases before they even became big. So offer them to sign a harsh confidentiality agreement and let them talk to reference clients who already trust you. In the end nobody can prevent a leak – it's more about trust than about legal agreements. So build rapport with smaller artists, always be reliable and trustworthy and then let word of mouth and references carry you to bigger artists.