Once you have learned a bit about the theorist and their contributions you will want to look at some of their work. Their work could be published in various types of publications.
Many theorists have written books on their work and provide explanation on how the work can be implemented in the field by others. These are primary resources by the authors. You won't have time to read them cover to cover. Review the table of contents to find the section or chapter containing the information you need. You can also review the index (if found).
Theorists may update their work from time to time which is way you find updated versions. Books take longer to publish which is why there will be a number of years between editions. Also, don't judge the theorists work by the date of the publication. These are considered primary resources presenting their thoughts at the time.
Research articles are published in peer reviewed journals. Here you will find the theorists as authors as well as other's writing on the use or thoughts of the approach. Research articles can be published much quicker than a book, therefore you want to take note of the date of publication. In these cases the you will want to look for the newest research.