The Telegraf package is installed using the package manager of pfsense. It is simple to install. All that is really needed after that is the location of your Influxdb (or whatever server), the database name, username, and password.
With this filled in and the settings being saved that is pretty much it, except…
I found that the Telegraf service was starting and stopping on its own. I looked over this and noticed there was a list of Telegraf outputs. I realized that I needed to select one. Telegraf didn’t need it selected in order to save, so that’s a bug. But when you do select the proper Telegraf output the service won’t stop and start repeatedly.
Ensure you have the Telegraf “Outputs” selected. If you are using influx then select the influx option.