Create a directory where to put the .AppImage files

juergen@tokyo:~$ mkdir ${HOME}/Applications && cd ${HOME}/Applications

Download the AppImage

juergen@tokyo:~/Anwendungen$ wget https://github.com/nextcloud/desktop/releases/download/v3.1.3/Nextcloud-3.1.3-x86_64.AppImage

Make it executable

juergen@tokyo:~/Anwendungen$ chmod a+x Nextcloud-3.1.3-x86_64.AppImage

Run it once

juergen@tokyo:~/Anwendungen$ ./Nextcloud-3.1.3-x86_64.AppImage

Setup autostart

juergen@tokyo:~/Anwendungen$ gnome-session-properties

Add Nextcloud to the Ubuntu launcher

juergen@tokyo:~/.local/share/applications$ vi nextcloud.desktop

Insert the following into the file and save it afterwards

[Desktop Entry]
Name=Nextcloud
Comment=Nextcloud
Exec=/home/juergen/Anwendungen/Nextcloud-3.1.3-x86_64.AppImage
Icon=/home/juergen/Anwendungen/Icons/Nextcloud.png
Terminal=false
Type=Application
Categories=Network

Make sure that an image Nextcloud.png exists at the correct location. (create one with 96×96 pixels).