When the documentation is difficult to understand it's useful to look at the existing tagnavi.config already on your player as it has multiple examples of different definitions. I had to do so in order to understand how to make myself custom search sub menus a while ago (I also found the documentation alone was slightly overwhelming) and by spending a few minutes again just now seeing how it's done I think the following will be useful.
Assuming you are trying to make an entry for the database main menu you can create a file tagnavi_custom.config like so:
#! rockbox/tagbrowser/2.0
%menu_start "main" "Database"
"User Rating Custom" -> rating -> artist -> year -> album -> tracknum -> title = "fmt_title"
which will add User Rating Custom to the database main menu and allow you to filter as intended. Tracks without a Year/Date tag get assigned to year 0.