What you must remember at all times is that a project can be saved only if you’re creating it while being logged in to your account. There is no way to save or restore a project that has been created without an account.
Once you’ve assured that you’re logged in, the rest is easy. The project is automatically saved with every change you make there. Just make sure you have a good internet connection in case you’re using the Web platform. Other platforms will save the project even in the offline mode. But synchronisation between different devices will only be possible once you connect your device to the internet.