I am aware that there are many post about this, but I honestly didn\'t understood any of it.
So, how do I install a kit for my Qt Creator (open source 5.7)?
You should decide if you are going to use the MSYS2 ecosystem or not. If you want to use MSYS2, you should uninstall the Qt software that you downloaded separately to avoid confusion. For the rest of this answer, I'll assume you are using MSYS2.
You can install these MSYS2 packages using pacman:
Then open up a MinGW-w64 32-bit or 64-bit shell using the appropriate shortcut in your Start Menu, and run "qtcreator" at the command line.
You can also use other build systems to build your software. I have used CMake successfully for building Qt applications in MSYS2, instead of Qt Creator.