Requirements
Your Notebook and Android devices should be in the same WLAN network.
Do not use a WLAN guest network. (might block direct traffic between devices)
Install App and Prepare For Sharing
Android: Install the app 'Screen Stream' via the Google Play store: Screen Stream
Android and Notebook: Connect your phone and your PC to the same Wifi. (important!)
Android: Open the newly installed app 'Screen Stream' on your android phone/tablet.
Android: At the top of the app you see the 'Device address', you will need this later. But first press the 'start' button in the app.
Android: If you are asked for permissions, press 'START NOW'.
Android: and Notebook: Copy (by hand) the 'Device address' as mentioned above from the app into your browser address bar.
Notebook: Select full screen.
Notebook: Now you can share your screen using another application on your Notebook. For example Jitsi or Adobe Connect.