Please start here: Beginner’s first steps - #46 by Kyle_Rankin, on how to use grim package. Simplest one would be (the one that I’m fine with):
sudo apt install grim
Open New Window from Terminal upper right corner and type:
sleep 7; grim
Now you have 7 seconds to flip to the screen that you need to save for you record. Above command puts screenshot directly into /home/purism directory, therefore you would want to use: sleep 7; grim $(xdg-user-dir PICTURES)/$(date +'%Y%m%d_%H%M%S_grim.png').
In my point of view @Sharon is the one that will master Librem 5 up to her pure satisfaction.