http://www.debian.org/doc/manuals/debia ... asics.html
The most important first step is to ensure the .DEB is compatible with your Linux Distro..
The second most important is ensure you have the dependencies
This is taken from a blog:
When you download packages for Linux Debian or Ubuntu they usually come in the .deb format and are installed automatically by your local package manager. If you download them from the internet instead of the repositorial way however you are given the .deb file and have to deal with it yourself. To install it, open a terminal, direct it to the folder where it has been downloaded to with cd and use the dpkg command:
dpkg -i random_name.deb