Child Themes
The best way to create a custom theme for Fusion is to create a child theme for the Fusion Base.
This will allow you to add custom style and functionality without modifying the code of the parent theme so that you can install updates without losing your customizations.
The WordPress Codex has a guide for creating Child Themes.
To help you get started, we have built the simplest possible Child Theme Framework; an ideal starting point for your custom child theme.