click on the 'Authoring' tab

click on the 'Assign Categories' link in the list on the left side of the screen

click on 'Add categories' for the option required

the next screen will display a tree of categories; to expand it, click on the '+' in front of a category name; this will show all the sub-categories which are grouped under a parent category

once you have found a category which you want to assign to the content item, click on its name which will enter it into the assigned box

if you wish to remove a category before saving, double click on it's name to do this

repeat this as many times as necessary; you can assign more than one category to the content item

once you have selected all the categories you want, click on 'Save'; this is very important, if you do not do it, your choice of categories will be ignored