To add the category to other categories, add each on a new line:
[[Category:Name of a category]]
[[Category:Name of another category]]
Do not add pages to the category here. To add pages to this category, you need to edit the pages separately. For more information on how to do this, see Wikipedia:Categories#Putting pages into categories.
Click Save page, and make sure everything looks okay. Once you have added pages to the category, you should see links to them listed under different letters.