Make sure you first have the latest version of ConfigurableFirmata installed in your sketchbook libraries folder (typically
/Documents/Arduino/libraries/ for Mac or Linux or
\My Documents\Arduino\libraries\ for Windows). The latest version is 2.10.0.
Next, select how you will connect to your client application. Typically this is a Serial connection at 57600 baud, but Wi-Fi, Ethernet and BLE are also available for certain boards and shields.
Once you have the connection type configured, select the core features and any contributed features your application requires.
Finally, name your file and click the Build and Download button and your custom Firmata will be downloaded.