Proceed as usual. Using the extension kickstarter will ease the process.
In your plugin php-file, add, just before the class declaration:
PATH_formidableapi is a constant pointing to the main class php-file of Formidable.
Add these lines in the main() function of your plugin.
First line of code create the blank object.
Second line initializes it, passing the reference to the current plugin and the path to the xml file as parameters.
There's still a line of php to add, the one that renders your application.
To the bottom of your main() function, add a call to render(), so that you have:
Now if the xml file you gave as a parameter doesn't exists, you'll have this display coming up:
Just copy-paste the generated xml in the file path you gave during the init, and you're done.