I am currently trying reproduce, for learning purposes only, an experiment on the procedural learning of unstructured categories. I have read the article reporting the experiment and some other references, but they are somewhat hard to understand for my current level of knowledge in some aspects.
For instance, I could not understand how to produce the reported stimuli for unstructured categories, the main thing I would like to learn how to do.
Is there any educational resources available on this matter somehow easier to grasp? I have tried really hard to learn through the mentioned works but perhaps I need to read something more introductory.
 Crossley, M. J. Madsen, N. R. Ashby, F. G. Procedural learning of unstructured categories. Psychon Bull Rev (2012), v. 19, p. 1202-1209. doi:10.3758/s13423-012-0312-0
 Ashby, F. G. Valentin, V. V. The Categorization Experiment: Experimental Design and Data Analysis. In: Wagenmakers, E. Wixted, J. T. Steven's Handbook of Experimental Psychology and Cognitive Neuroscience. Vol. 5 (Methodology), 4th ed.