My main questions are: how do you realize this change? Do you try to find contracts before leaving your day job? Where/How do you find work?
I know the most common platforms but are there some rules you should follow on there?
Do I have to suck it up and offer my service for "cheap" in the beginning? Or should I only take jobs for my self-defined minimum even if it's my first?
What else should I consider?
The public websites for contract work are not great places to look unless you're okay taking the lowest common denominator jobs.
Your personal network is the best place to start. You need to start reaching out to contacts and offering your services for hire. If you're uncomfortable with this, you're not a good candidate for freelance work.
Some people get lucky and find a constant stream of niche work from their network right away. Others will have to work hard to find each additional contract. It will get easier over time as you build a reputation and expand your network, but it will take work to get there.
I would shy away from offering your services for cheap if you can. It can be a good thing to do when you are just getting started as developer and don't have a portfolio yet. Given that you have been building things for 7-8 years though, I would guess that you have some examples of things you have built that you can share with prospective clients?
If you have the capacity to try out freelancing on top of your day job, I think it is a great way to validate if you have the appetite for it and earn some extra money while doing it.
If you can start freelancing with projects lined up you won’t have to start out trying to get work, which will get demoralizing fast.
Don’t expect to get much from the popular platforms like Fiverr and Upwork. While it’s possible to get good contracts there, those sites are (a) full of low-paying short-term gig work, and (b) full of low-balling competition. Your need to aim higher because competition is always most fierce at the low end. Instead look to past employers, professional contacts, friends. Focus on building long-term relationships rather than churning through online piece-work boards.
Position yourself as a specialist in a niche that has business value (i.e. e-commerce security expert) rather than with a laundry list of languages, frameworks, stacks. Know your customer — no small business needs 2,000 more lines of Javascript by next month but lots of businesses need specific business problems solved.
I’ve been freelancing for over a decade. I wrote some opinionated advice on my web site (no ads, no paywall, no affiliate links).
https://typicalprogrammer.com/how-to-start-freelancing-and-g...
There are some other possibly relevant articles there too. Good luck.