GENERAL:  Garmin claims you can create Custom Maps in five easy steps (HERE).  'Your mileage may differ,' but using Google Earth to create Custom Maps was a time-consuming pain for this reviewer.  For the serious hiker, it seems that the USGS 1:24,000 DRG topo maps would be the most useful and legible on the GPS screens.  Google Earth isn't especially helpful in converting these maps to the required .kmz maps needed for the Garmin units.

Eventually I came across G-Raster which has an ability to import the georeferenced DRG .tif files and export them as a series of smaller georeferenced .kmz map files.  The DRG GeoTiff feature is fully free; you only need to pay $5 to allow you to input KMZ image overlay files created in Google Earth and tile them for export, and even then only for images larger than 1500 x 1500 pixels.

PROGRAM SETUP:
Exporting the data files as .kmz files to the Garmin unit is easy as 1, 2, 3.

NOTE: This procedure has been updated to incorporate the ability to de-collar the DRG maps for seamless map connections in update version. 2.6.  One must update the complete install (and not just the 'fixer') to get the latest Help files explaining the new feature.

1.  Open a Georeferenced "USGS 1:24K NAD-27 Topo Geo Tiff" (DRG) map from the drop-down menu.
2.  Auto Rescale will reduces the number of Tiles (maps) from 36 to 25, allowing one to upload four DRG maps to the GPS unit.
     The number of "Tiles" (maps) is shown in the Info area to the right.
3.  Create a KMZ file.  The KMZ file will appear as a .kmz extension to the original filename in the source folder.
     Upload this file to the GPS \Garmin\CustomMaps.  NOTE: You can follow the progress of the program in the DOS prompt icon in the tray.

RESCALE:
Apparently G-Raster by default creates 40-something maps per DRG map WITH collars.  Therefore only two DRG maps can be viewed by the Garmin at a time due to a 100-map limit.   Setting a Rescale Factor of "1.5" produced only 20 maps/KMZ and a usable resolution of 3.67m/pixel (below), allowing uploading five DRGs.  Loading the de-collared maps and using a Rescale of 1.3 cuts the Tiles down to 16, so that you can fit 6 de-collared topos on your Garmin.  One should experiment with Quality and Rescale to suit their own situation.

SOLVING THE BORDER PROBLEM IN OTHER WAYS:
-Not the one in Texas. The USGS Seamless Server, MicroDEM, and USAPhotoMaps can remove the collars which allows one to upload more maps.

I have had the best luck with USAPhotoMaps, because not only is it a great GPS map and aerial photo display program, but downloading USGS 24K topo maps requires you only to push the F (Fill) key!  The program has a feature to combine all the small geo-referenced (200 x 200 pixel) cells into one "BIG jpeg" file that G-Raster can read (from the drop-down list of supported file types.)  I use a Quality of 80 on these files, because the 24K ones are at 4m/pixel instead of the usual 2.5m/pixel from the DRG maps.

Now don't get lost out there -youall!
jack yeazel