Comment 8 for bug 1647400

The easiest way (imho) to enable proposed is:

enable proposed:
$ sudo sh -c "echo 'deb http://us.ports.ubuntu.com/ubuntu-ports $(lsb_release -cs)-proposed restricted main multiverse universe' >> /etc/apt/sources.list.d/proposed-repositories.list" && sudo apt update
$ sudo apt update

and for completeness to disable proposed:
$ sudo rm /etc/apt/sources.list.d/proposed-repositories.list
$ sudo apt update

before installing you may check if the packages you are looking at is really coming from proposed:
$ apt-cache policy <package>

But you can also use packages from proposed w/o this, just by using:

install package from a special (-t = target) repository, like from proposed:
$ sudo apt-get -t $(lsb_release -cs)-proposed install <package>
or:
$ sudo apt-get install <package>/$(lsb_release -cs)-proposed