Adding categories to Pages

Hi
I have created a training website in Goran theme at https://www.3ptraining.com.au I now wish to create categories for each of the pages. There is plenty of explanation of how to create a category when writing a post. however my pages are not posts and this has to be retrospective. I can add categories to the site, but how do I link a specific category to a specific page? Each page is a lesson topic/theme so categories need to be linked. Would really appreciate some guidance if anyone knows ??

3PT