Upon starting the program, you get a mostly blank window, with three buttons enabled (besides help).
Other than that, the program will look pretty empty. Don't panic. Once you open a BIQ, more stuff will appear. By not showing up right away, the program starts much quicker.
Once you've opened a BIQ, a number of tabs will appear. See the index for information on specific tabs. You'll also have several more buttons enabled; they are described from links on the index as well.
In general, to add items, right-click on the list of items, and choose Add. In a few cases, there are visible "+" buttons to use instead. If neither one of these exists, you probably can't add an item - for example, adding terrains is not possible. You can delete items, where possibly, in the same fashion. You cannot currently rename items.
The editor will automatically save your file, by default, every 4 minutes, and keep 10 backup copies. You can change this in the settings.
Once you've made your desired changes, click the Save button to save your scenario, and then test it in-game!
Currently, the editor only supports BIQ files that:
Additional, opening more than one file is not officially supported, except for the BIQ Compare (BIQC) functionality. If you wish to edit multiple files at once, starting the editor more than once is highly recommended.
Finally, the editor is still a work in progress. There are possibilities for improvements, and you're welcome to suggest more at the forum thread. At least one whole functionality area has already been added directly because of this, not counting smaller, easier improvements.