before do anything , you need to know the requirements to automate the integration between Nagios and Puppet.
You will need to make sure that your puppet working with some external config like puppetdb.
if you are not using it yet.
1 - Install some external config like PuppetDB or directly on mysql or postgresql.
2 - Configure the puppet server to use PuppetDB or some other external config.
after external config is done.
3 - create a puppet class for a nagios server and a class to export the informations from the nodes.
Here is an excellent getting started with nagios and puppet.
you can check this article for some advanced tips:
and the most recommended is the book Pro Puppet , there are a topic about external config and an example to how to integrate and automate nagios with puppet.
Have a lot of information about it on the internet.