First steps for a brand new installation:

1. Edit /usr/share/check_mk/modules/defaults and make sure all the relevant fields match your Nagios installation. Specifically:

nagios_objects_file         = '/etc/nagios/objects/check_mk_objects.cfg'
nagios_command_pipe_path    = '/var/spool/nagios/cmd/nagios.cmd'
check_result_path           = '/usr/local/nagios/var/spool/checkresults'
nagios_status_file          = '/var/log/nagios/status.dat'
nagios_conf_dir             = '/etc/nagios/objects'
apache_config_dir           = '/etc/httpd/conf.d'

2. Add check-mk's template and objects files into /etc/nagios/nagios.cfg:

cfg_file=/etc/nagios/objects/check_mk_templates.cfg
cfg_file=/etc/nagios/objects/check_mk_objects.cfg

3. Edit /etc/check_mk/main.mk and add the hostnames / IPs of the machines you want to monitor.

4. Run check_mk -I and check_mk -O, this will run an inventory check and rebuild your Nagios configuration files.
