Creations are saved automatically with every change you make. You can resume past creations from the ‘Creations’ tab on the bottom of the navigation bar as long as you stay logged in and you don't log out.
If you log out, you'll lose all creations from 'Creations' tab. However, if you have added those creations to the basket, they'll be saved there.
Note: if you’ve started a creation on the website, you cannot edit it on the app and vice versa as these platforms are not connected.