Thread:ADour/@comment-25414732-20151206092122/@comment-4651179-20151206184655

When you go to the edit mode of a page, you'll see a thin grey bar to the right of the edit box (next to the scrollbar). This bar as a diminutive black arrow/triangle pointing to the left.

Click on this bar and you'll see it has extended into a whole new options bar. This bar will have different sections pointed by bolded text. One of them is "Categories." In case it's not already extended, click on the "Categories" text. Now you'll see two boxes. One small which says "Add category..." and another which displays categories already present in the page. Now you only have to write down the category you want to add and type enter, and it's done.