The bare bones minimal would be
sudo apt-get install xfce4
If you do not install a log in manager (slim, gdm, light-gdm, etc) you will not have a graphical login screen, and in that case you can either startx
or xfce4-session
Depending on how much or little you wish to configure, you may want to go with lightdm ;)
If you need assistance configuring your xfce4 session (from the command line or with a log in manager) post back.
You will almost certainly wish to install additional fonts and additional xfce packages. Many of the extras are "plugins" , ie "xfce4-foo-plugin"
Either search for the packages, or if you prefer, install the "goodies" package
http://packages.ubuntu.com/precise/xfce4-goodies
Additional packages to consider (IMO) might be firefox, synaptic, squeeze, xfburn
Text editors , personally I prefer gedit as it has a number of features, but leafpad or mousepad are more traditional / lightweight (but scant features).
The following would give you a nice (IMO), minimal setup, adapt the packages as you wish.
sudo apt-get install xfce4 xfce4-goodies xfce4-mixer gedit squeeze synaptic xfburn lightdm audacious audacious-plugins
Other plugins listed here
http://packages.ubuntu.com/cgi-bin/search_packages.pl?keywords=xfce&searchon=name&subword=1&version=all&release=all
Add in a few fonts and multimedia as required and you would be good to go.
Best Answer
For minimal install, you have to choose the normal install, and during installation it offers the choice of minimum install.
In fact, everything is installed first and in the end, it removes most of the things necessary to make Ubuntu run. For details, have a look at: https://www.omgubuntu.co.uk/2018/02/ubuntu-18-04-minimal-install-option