Feeling really stupid, right now, but the title says it all:
How do you start the QtDesigner?
I've installed PyQt5 via pip and I believe to have identified the directory it's been installed in as
C:\Users\%username%\AppData\Local\Programs\Python\Python36\Lib\site-packages\PyQt5
Now what? There are a lot of .pyd files, some .dll's, too, but nothing executable (well, except a QtWebEngineProcess.exe in ...\site-packages\PyQt5\Qt\bin, but that doesn't sound like what I'm looking for.
I struggled with this as well. The pyqt5-tools approach is cumbersome so I created a standalone installer for Qt Designer. It's only 40 MB. Maybe you will find it useful!
The Qt designer is not installed with the pip installation.
You can either download the full download from sourceforge (probably won't be the last pyqt release, and might be buggy on presence of another installation, like yours) or install it with another (unofficial) pypi package - pyqt5-tools (pip install pyqt5-tools), then run the designer from the following subpath of your python directory -
...\Python36\Lib\site-packages\pyqt5-tools\designer\designer.exe
If you are working in python virtual environment, in the command window
>>qt5-tools designer
can open designer window.