The easiest way would be to use an existing POI and then modify it.
- Go to: http://garmin.gps-data-team.com/
- select your country
- choose custom POI
- Click on the edit POI button from the bottom menu
- Upload existing POI file (any file that closely resembles the one you want to create)
Use the online tool to write/update the desired information.
I made a POI file from scratch of Greece overnight camping sites. For some reason, only about 8 of the places appear on my GPS when I do the upload. I cannot figure out what I have done wrong.