" But if the location is Dallas county, which is the correct entry?"
I use Dallas County, Texas, USA. I want to be sure when I (or anyone else) looking for that the name beside the state is a county or a subdivision within the county, so I always use County with the county name. FTM recognizes the text string county as part of the name and will resolve with the county in the name.
"Can anyone point me to a reference that has this kind of basic "how to" info"
You have the PNA built-into FTM - which is very imperfect.
The first place I often look is Wikipedia, because it often discusses the names of that place historically; as well as other places spun-off from it.
I also often look in GenWeb and county history books at googlebooks.com and archive.org.
To illustrate the problems that come with FTM (besides not having historical names and some names can be found in the Bing maps and not in the PNA) are:
Los Angeles. The following areas will be resolved as "area, Los Angeles, California, USA" within the PNA, but:
Marina del Rey is an unincorporated area in the county (not in the city)
Venice and Sherman Oaks have been incorporated in, and are now, neighborhood names in the City of LA
Beverly Hills and Santa Monica are separate cities in the County of LA.
Also, the five part names in areas of villages and hamlets in places like New York resolve to four parts, often leaving out the name of the town the village or hamlet is in. One has to manually override the four parts and add the town to the PNA name to get it right.
So, we can only do what we can do with what we know.