how to install a debian package ….?

You need to use dpkg package manager from shell/command prompt. dpkg is a tool to install, build, remove and manage packages. dpkg itself is controlled entirely via command line parameters. For example -i use to install .deb file.

How do I install .deb file?

To install package called package.deb type the following command:
Go to directory where package.deb is kept. For example if it is in /tmp directory:
$ cd /tmp
Type the following command:
$ sudo dpkg -i package.deb