Comment 31 for bug 1783252

The (imho) easiest way to install packages from proposed is like this:

First of all enable proposed:
sudo apt install software-properties-common
sudo add-apt-repository "deb http://us.ports.ubuntu.com/ubuntu-ports/ $(lsb_release -sc)-proposed restricted main"
(depending on the Ubuntu release you might need to do an "sudo apt update" now to update your archive indices.)

Next step could be to check for any available packages (and from which archive they come from) like this:
apt-cache policy <package>

Now have (at least) the following three options:
1)
in case the package you are looking for has the newest version (which is very likely the case) just do:
sudo apt-get install <package>
2)
Provide a specific package version during installation (update):
sudo apt-get install <package>=1.23.4.5.67
3)
Point to a specific archive where the package you are going to install should be taken from:
sudo apt-get -t $(lsb_release -cs)-proposed install <package>

[Replace <package> with a valid package name.]