Make sure you have a MapSource product or Trip & Waypoint Manager (that came with your GPS) installed in your PC first. Update it to the latest version found in Garmin's website.
1. If you already have some maps installed then go to step 3, otherwise download the S'pore & Johor MapSource Installer or any other installers from Downloads section.
2. Run the installer to install the maps (Do not change the default path).
3. Launch MapSource and select View>Switch to Product (or use the Product list 
dropdown list in the tool bar) to choose the map you would like to display.
Go to View>Map Detail and select the amount of detail you want to see in MapSource.
4. Select Tools>Map (use the M key or the Map Tool 
in the tool bar just below the menu) and click anywhere on the map to select it (the map will highlight pink and its name will appear under the Map tab on the left panel).
5. You can repeat steps 3 & 4 to select additional map tiles to upload to your GPS receiver.
6. Use Transfer>Send to Device or Send icon 
to upload the selected maps to your GPS device.