The official city maps for Garmin are inexpensive, and awesome. But if you are visiting a place one-off or you are just being a cheapskate :-) it's worth considering garmin.openstreetmap.nl where you can download detailed and free routable maps for Garmin devices. I've been using the Victoria and NSW Australia one for some time now. And they are awesome. I have tried a routable map for Melbourne on my Garmin Montana, and it works very well in both road navigation (Nuvi like) and geocaching (Oregon like) modes.
4. Download each map in turn. It is a ".img" file, and named just as a series of numbers. It's probably a good idea to rename each of these to something more meaningful. So instead of 5566733.img - rename it to singapore.img
5. Copy each of these .img files to either the Garmin directory on your Garmin GPS or to a directory named "Garmin" on your memory card of your GPS.
6. Make sure your new maps are enabled in the map settings of your Garmin GPS. You should now have the maps available for use on your Garmin GPS.