Kurt Lewin published Field theory in social science in 1951. You can find a copy on the Open Library and read it there. He develops the ideas that later become known as the 'Lewin's three step model' in the chapter entitled 'Frontiers in group dynamics' which starts on p.188. Reference it as follows:
Full reference: Lewin, K. (1951) Field theory in social science. New York: Harper & Brothers.
In-text citation: (Lewin, 1951) or Lewin (1951).
A straightforward introduction to Lewin's 3 step theory, including a diagram, can be found in Making sense of change management by Esther Cameron and Mike Green. University of Worcester students can access Cameron and Green here. If you find this easier to use, reference as follows:
Full reference: Cameron, E. and Green, M. (2019) Making sense of change management: a complete guide to the models, tools and techniques of organizational change. 5th edn. London: Kogan Page.
In-text citation: Lewin's 3 step model (1951, cited in Cameron and Green, 2019, p.115)...
