There are only two caveats in this case. First, QML files that are to import anything from the library must first import 'MyLib 1.0' before importing anything else ('MyLib.Rectangle', 'MyLib.Circle', etc.). This is so that the plugin is loaded initially to populate the other imports. Secondly, this will produce the following message when the plugin is loaded:
Module 'MyLib' does not contain a module identifier directive - it cannot be protected from external registrations.
However in my case, this is not an issue and can be safely overlooked.
Qt Designer is included in Qt sources and works only for UI files. Qt Creator (the IDE) is a separate application and a separate project. You need to install or compile it first, for example from qt.io/downloads.
Once you have Qt Creator installer, you need to open the settings, add your Qt Version, then add a so called Kit (which is Qt Version + compiler bundle), and then you are ready to go.
If you want to avoid all that hassle, use the Qt installer from the link above - it will install Qt, Qt Creator, and set everything up for you, completely automatically.