Go to https://console.cloud.google.com/apis/dashboard
Select a project -> New Project -> Create
Click "Enable APIs and Services" -> Search "Photos" and select "Photos Library API" -> Enable
Goto "OAuth consent screen" tab -> Select "External" and create -> Fill the mandatory fields. Be sure to fill "photos-kodi-addon.onrender.com" as authorised domain -> Save and Continue
Click "Add or remove scopes"
Select the following scopes
./auth/userinfo.email
openid
./auth/photoslibrary.readonly
Fill other mandatory fields as per your choice and "Save and Continue".
Click "Add Users" and enter all the accounts which you want to use with the Kodi addon.
Save and Continue -> Back to dashboard
Open "Credentials" tab.
Create credentials -> OAuth Client ID -> Web Application
Set "https://photos-kodi-addon.onrender.com" as Authorized Javascript Origins
Set https://photos-kodi-addon.onrender.com/auth/google/callback as Authorized redirect URIs -> Create
Copy these client ID and secret to the Addon settings.