To download charts, open the Main menu and select Offline Charts. You will now see an overview map of your current location along with several squares covering parts of the chart.
Downloaded areas are shown as green
Areas that are currently downloading are shown as blue
Selected squares are shown in grey
To download new charts:
Pan the map to your desired location, and a number of grey squares will appear at the center of the screen.
Press on Download to begin downloading the highlighted areas.
The selected areas will turn blue and show a progress indicator. Once the download is complete, the area will turn green.
You can download multiple cells at a time by panning the chart to a new area and pressing Download again.