This is a sample of the GPS chart for NUVI. It is specifically for use on SD cards.

Unzip the file to a convenient place.

Take a clean SD card, make a folder called "Garmin" ("Map" on newer Garmins - you can actually make both  - just to be sure) and place the unzipped file into the new folder/s.  Its called gmapsupp.img.

You may need to tell your NUVI to use pedestrian mode, and point it to the correct chart using the settings menu. The sample map is an area from Oughterard towards Inchagoill.