I'm assuming you've installed Lighttpd with the package manager of your distribution of choice. Next you need to enable the fastcgi module.
If you're using Debian or a variant (like Ubuntu) you can use the lighty-enable-mod script:
This will create a symlink from /etc/lighttpd/conf-available/10-fastcgi.conf to /etc/lighttpd/conf-enabled/10-fastcgi.conf. Open this file in your editor. If this doesn't work for you simply edit the main configuration file /etc/lighttpd/lighttpd.conf.
To enable the module you need this line:
server.modules = ( "mod_fastcgi" )
If you find any fastcgi.server portions in the configuration comment them out, we won't be needing them. Now add a fresh portion:
fastcgi.server = ("" => (( "bin-path" => "/path/to/app.rb", "socket" => "/tmp/nitro.socket", "check-local" => "disable", "max-procs" => 1, "bin-environment" => ("RUBYOPT" => "-rubygems -I/path/nitro_repo/script -rlib/glycerin") )) )
sudo /etc/init.d/lighttpd force-reload
and you should be all set. Keep an eye on /var/log/lighttpd/error.log in case stuff doesn't work right.
Adapt bin-environment to whatever is necessary on your site to get nitro running. I'm assuming you're using the repo version. Otherwise a simply "-rubygems" might suffice.