Making maps uploadable by sub-section

[NeuroDoc]

I own some garmin maps, and have made some small, personal maps. I am in the process of making  a larger, state map, but after I create it and load it into mapsource, I want to be able to select subsections (by county) of the map to load onto my device (like I can select states with the Garmin City Navigator maps).

How do I do this?

Sorry if this has been discussed elsewhere, my search was unsuccessful.