• Find out you home latitude and longitude in degrees.
  E.g. visit google-maps, left-click on your home location and read off the coordinates in the bottom of the page. The first number is the latitude, the second the longitude.
  
• Get a KML file with your missing tiles. Visit veloviewer, and download the KML file from your summary page.
  
• Create a map-string. Visit the map-string-generator, enter the home coordinates and upload the KML file. Click on "go!"
  
• Verify that the created map is, what you wanted. the home tile is in the center, visited tiles should appear in green. Optionally, you have the possibility to edit the map by left-clicking on tiles, that you wish to change. After the editing, click the "go!" button at the bottom, to update the map-string.
• Select the whole string and copy it into the clipboard by pressing "Control-c".
  
• Start garmin express. Install (if not yet done) the EveryTile app from the App-store. Go to the settings of EveryTile
  
• Enter your home coordinates and paste ("Control-v") the map-string into the approriate field. Click "Save".
  Note: the map-string is ignored (and instead the old persistent data on the device is used) if the home coordinates have not changed. To enforce an update, change one of the coordinates slightly, e.g. from 7.151 to 7.1511.
  
• After synchronization with your device you should be ready to explore!

disclaimer