I agree that the question is clear — i.e. you can tell what it's asking, but that isn't the problem here. The issue is that any complete answer to that question would be unreasonably long; the question essentially requires a step-by-step tutorial... and that's something that this site isn't really built to handle.
Too Broad generally means that your question is too wide in scope for this site. That isn't immediately obvious in some cases if you're not familiar with how Stack Exchange works, but a handy rule of thumb is that if you'd expect a complete answer to be more than a few paragraphs long, it's likely too broad.
See also the help center:
too broad - if your question could be answered by an entire book, or has many valid answers (but no way to determine which - if any - are correct), then it is probably too broad for our format
We close these questions because they don't work in our format, and the only way you can really answer them at all is with a link (as you did), or by providing excessively brief answers which don't really solve the author's problem.
If you were facing a specific problem while setting up your magic mirror, then that would make a great question for the site, for example:
I'm trying to make a magic mirror with a Raspberry Pi 3, and I have connected a [insert monitor model] to the Pi, but this isn't working because [insert problem here]. The Pi works fine with other monitors but not this one. What can I do to fix this?
That would be a specific problem that could be answered. But questions which ask how to create a rather big project would need unreasonably long answers, and aren't good fits to this site. The best solution for the author here would be to search for an existing tutorial, or perhaps ask on a more discussion-oriented site like Reddit (where indeed there are how-tos and other interesting links available).
As I say, getting a handle on exactly what makes a question 'too broad' takes a bit of experience with Stack Exchange. I hope you'll participate here in future and get to know the system a little better... and eventually you should be able to "know it when you see it".