1. Open maps app and click on the side bar(3 horizontal lines at the left top)
2. Select 'Offline Areas' option,
3. Select the '+' at the bottom right
4. choose an area. you can choose a very big area and download,
1. But it has a limitation, For eg. it cant get the whole of TN in one offline area, so i did choose multiple offline areas that collectively covered the entire TN.
2. The download size would be easily more than 100/200 mb, it will infact show the estimated data size when you select the area. So do this when you are on good internet speed and be aware of how much it will cost you, home wifi might be a good option
3. Please remember that GPS will work without internet, so now anywhere you roam around TN with maps navigation offline, ofcourse no live traffic data.
4. Offline areas have an expiry timeline of 30 days. You have to update them.
It seems this feature was released only last month, rolled out 10th of Nov, 2015 https://googleblog.blogspot.in/2015/11/navigate-and-search-real-world-online.html