Aptitude is preferred, but other approaches can be used if desired.

First, update the package index. The package index is essentially a database of available packages from the repositories defined in the /etc/apt/sources.list file and in the /etc/apt/sources.list.d directory.

sudo aptitude update

Now install the package.

sudo aptitude install package_name

If Experimental was enabled in the sources.list file, the corresponding packages will almost never be installed because their default APT priority is 1. This is of course a specific case, designed to keep users from installing Experimental packages by mistake. The packages can only be installed by typing

aptitude install package_name/experimental

Users typing this command can only be aware of the risks that they take. It is still possible (though not recommended) to treat packages of Experimental like those of other distributions by giving them a priority of 500.

This is done with a specific entry in /etc/apt/preferences:

/etc/apt/preferences

Package: *
Pin: release a=experimental
Pin-Priority: 500