The default name for a new style is <username>-<view type>-new.
Since the style creation page will not overwrite an existing style, you will need to change the name of your new style if you would like to create additional styles for that view. Until you change the name of the style from the default name, you will be unable to create new styles for that view.
You can change the name of your style on the Edit Style page (http://www.insanejournal.com/styles/edit.bml), under "Style Description". Under "Style Options", you can choose to make the style Public, which affects whether or not another InsaneJournal user can see the coding that makes up your style (and possibly use it).
You will need to save your style for any changes to take effect.
Once you have created a custom style, you will first need to select that style for your journal. Go to http://www.insanejournal.com/modify.bml and under whatever view you made it for, choose the style from the drop-down menu and Save Changes.