Cannot install gadget without model assertion error

I’m starting to try snap developments it’s a python app for pi3 with access to serial port.
I understand I have to create a gadget.yaml file and a assertion.model.
the gadget.yaml is stored on the root of my project, snapcraft.yaml is created in snap directory… but where do I store gadget file?
the whole project directory setting is not clear for me

To build a modified gadget for pi3 (make sure you have snapcraft installed):

   ogra@anubis:~$ git clone
   ogra@anubis:~$ cd pi3-gadget
   ogra@anubis:~/pi3-gadget$ <edit snapcraft.yaml and add your additional interface>
   ogra@anubis:~/pi3-gadget$ snapcraft
   Snapping 'pi3' ...
   Snapped pi3_16.04-0.5_armhf.snap

Now you find a modified gadget snap in the pi3-gadget dir.
If you later use ubuntu-image to create an image, you can use the --extra-snaps option with the path to this snap file as argument. In your model assertion, make sure there is the line “gadget: pi3” which tells ubuntu-image how the gadget it should use is called.