What if the style you need is not available?
It is best to start with an existing referencing style which is similar to the one you want. Browse in the categories in Style Manager. Start with a similar style to what you want, rename it and edit it per the information under Editing/Customising styles.
You can also build/create a totally new style from scratch. Instructions can be found in the first pdf above (Creating EndNote Styles).
The second pdf (EndNote Windows Style Editing Guide 2014) contains more detailed information and instructions on editing and creating styles for EndNote.