An outline of this procedure is given in the following paper:  
Max R. Taylor [J. Appl. Cryst. (2000). 33, 975-976]

Max R Taylor
Department of Chemistry, 
The Flinders University of South Australia, 
GPO Box 2100, Adelaide, 
Australia, 5048

E-mail: Max.Taylor@flinders.edu.au

Web-site for spread-sheets:
http://www.ccp14.ac.uk/ccp/web-mirrors/max-taylor-fourier-spreadsheet/