1. Install nagios

2. Edit

contacts.cfg
hostgroups.cfg
hosts.cfg
services.cfg
nagios.cfg
checkcommands.cfg

Reference, files

Essential is that you delete or comment whathever you do not need from the
configureation files. All you need is your contacts, to get mail,
hosts and hostgroups for you two nodes. Services, the one service we
are going to monitor, samba. And the command that should be issued in
case of a failure, Reference checkcommands.cfg.

Add your /usr/lib/nagios/eventhandlers/failover script