After downloading nessus, it can be installed using below command
#dpkg -i nessus_package_name.deb
Start nessus using below command
You can access nessus on TCP port 8834 using localhost or hostname in the place of ip address, say localhost:8834. Login to nessus using the credentials you configured while installation.
After successful logging, you will see below screen.
Select the scan which interests you, will take you to below page where you need to configure IP Address, on your left you can customize Reporting info, Scan info etc. Don't click on save if you are not done with customization, once you click save Nessus will start the Assessment you have selected, in my case it was "Basic Network Scan".
To create custom policy we can click on Policies tab on top of the page.
To configure credentials for SMB, SSH, FTP, HTTP etc you can click on Policies on top of the page once you successfully login, then click on Credentials.