1. add repository : 'echo "deb [signed-by=/usr/share/keyrings/sus-archive-keyring.gpg] https://apt.seffner-schlesier.de/ stable non-free" > /etc/apt/sources.list.d/sus.list'

2. download key : 'wget -O- https://apt.seffner-schlesier.de/sus-apt-key.asc | gpg --dearmor | sudo tee /usr/share/keyrings/sus-archive-keyring.gpg >/dev/null'

3. update apt database : 'apt update'

4. install packages ...