Install Zettlr

The Zettlr software can be installed using the official instructions for Debian/Ubuntu:

curl -s --compressed "https://apt.zettlr.com/KEY.gpg" | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/zettlr_apt.gpg > /dev/null
sudo curl -s --compressed -o /etc/apt/sources.list.d/zettlr.list "https://apt.zettlr.com/zettlr.list"
sudo apt update
sudo apt install zettlr

See also: Install Zettlr in a Qubes template