Since we have to search the 1940 Census records via Enumeration District, how do you find out what that district is for your ancestor?
Try locating your ancestor in the 1930 census, and look at the top of that page for the town's Enumeration District number. (It will look something like this: ED-57) Take that information to the 1940 Census search page and enter the ED number in the appropriate place. Start scanning the pages. Be patient!