To add a subcategory all you need to do is go to your post. The box you see below will be on the right hand side of the post you are writing in WordPress, obviously with your categories and not mine;

So to my category 'Reviews - On Marketers" I have added 'Jim Cockrum'. Yes, I do know this is not my Steampunk site, that is still under construction.

To do that all you do is click on '+ Add New Category' and you will see this;

The Parent Category will magically appear, this will be your main category like this;

Then I choose my parent (not literally of course) which is Reviews - On Marketers, like so;

As you can see I have typed in Jim Cockrum and when I click on 'Add New Category' Jim Cockrum will be added under 'Reviews - On Marketers' easy peasy. That is how I got the picture at the top.

Now let's have a little chat about how useful categories are for your customers...