Here's how he did it:
Until recently, the only long time series of house prices for the United States had been compiled by Shiller (2005). Shiller constructs this series by splining together available house price data from 1890–1934 from Grebler, Blank, and Winnick (1956), the home-purchase component of the CPI-U from 1953–1975, the OFHEO from 1975–1987, and the Case-Shiller-Weiss index from 1987–2005. To fill in the gap, Shiller constructs an index of house prices from 1934 to 1953 by compiling data on the sales price of houses from five major cities based on newspaper advertisements. These data, after adjusting for consumer price inflation, show almost no trend increase in house prices until about 1997, leading Shiller and others to conclude that the boom to house prices from 1998–2006 is historically anomalous.