Download Ubuntu Server and install it.

If you have a proprietary graphics card, then add the following PPA:

$ sudo add-apt-repository ppa:oibaf/graphics-drivers

If you want to install the latest version of the Cinnamon Desktop Environment, then add the following PPA:

$ sudo add-apt-repository ppa:embrosyn/cinnamon

If you want to install the latest version of Kodi Media Center, then add the following PPA:

$ sudo add-apt-repository ppa:team-xbmc/ppa

If you want to install Syncthing, then enter the following commands:

$ curl -s https://syncthing.net/release-key.txt | sudo apt-key add -
$ echo "deb https://apt.syncthing.net/ syncthing stable" | sudo tee /etc/apt/sources.list.d/syncthing.list

Now update your system:

$ sudo apt-get update && sudo apt-get -y upgrade && sudo apt-get -y dist-upgrade

Install any media codecs that you like:

$ sudo apt-get install ubuntu-restricted-extras libdvdnav4 libdvdread4 gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly libdvd-pkg
$ sudo dpkg-reconfigure libdvd-pkg

Install all of the packages that you like:

$ sudo apt-get install cinnamon lightdm ubuntu-wallpapers firefox filezilla brasero gimp thunderbird eog evince totem simple-scan software-properties-gtk software-properties-common kodi kodi-pvr-hts syncthing gdebi libreoffice gnome-calculator nautilus-admin

You can enable Network Manager by editing the netplan configuration file in /etc/netplan to look like this:

network:
  version: 2
  renderer: NetworkManager

Then run the following commands:

$ sudo netplan generate
$ sudo netplan apply

If you have an HP printer you can now download the latest version of HPLIP and install it:

$ sh hplip-3.19.5.run
$ sudo hp-plugin -i

Now reboot and you should be able to log into your new custom desktop environment.