When you run the command name FF or FireFighting for the first time, the Scale dialog will be shown on the current drawing. After you specify the right scale of the drawing then the main dialog will appear immediately. You can see the preview of this main dialog in the screenshot. All you need to do is to choose the desired layer name from the list. 

You need to specify the scale from the scale dialog which appears only once. Next time when you call the program or close and reopen the same drawing, the program should pass over the Scale dialog as long as the scale is already set in the drawing.

You need to select the desired pipe size button after specifying the current or a specific layer name from the popup menu available in the dialog. Now you will be ready to start drafting the Fire Fighting Pipe along with the related fittings.

The 'Change Scale' button changes the scale of the Fire Fighting Pipes & Fittings if you have chosen the wrong scale from the first dialog that appeared at the start of the program.

Finally, you can hit the Activate button in the main dialog box to get the permanent full version of the program. Follow the steps from the dialog that appears right after hitting this button.

The installer that ran when you downloaded this app/plug-in from Autodesk App Store will start installing the app/plug-in. OR, simply double-click the downloaded installer to install the app/plugin. You may need to restart the Autodesk product to activate the app/plug-in. To uninstall this plug-in, exit the Autodesk product if you are currently running it, simply rerun the installer, and select the "Uninstall" button. OR, click Control Panel > Programs > Programs and Features (Windows 7/8.1/10/11) and uninstall as you would any other application from your system.