1. Go to Flickr website and connect to your account
2. Go to the API keys page
3. Click Get another key
4. Choose Apply for a non-commercial key
5. Enter a name and a description and check the two checkboxes before submit
6. Now copy the Key and the Secret to Flickr2Piwigo configuration
7. Click Edit auth flow for this app
8. Enter a desciption again, past in Callback URL the above URL and click Save changes
9. You're done!