The Island Editor is freeware and was given life only by pure pleasure in programming.
As quick reference we absolutely recommend the Tutorial by Guardian. The basics for the creation of own islands are explained there step by step, and better than this can be achieved by a manual. But even advanced island designers may still find there new possibilities and hints for efficient work with the Island Editor.
Here is a screenshot of the editor combining the islands "Deathvalley" by Joe and "Xidani" by Budgie: