How to get User-ID

You need an account in Zotero (be a user)
Go to: https://www.zotero.org/settings/keys
The six digit ID will be displayed in the first line right before the phrase "Your UserID for use in API call is: XXXXXX"
Copy and paste the six digit ID in the user settings of Biblioretratos. This is only done once.

How to get Zotero private key ?

Go to https://www.zotero.org/settings/keys
Click in "Create a new private key
Fill the information as it is shown in the image and finish by clicking in Save key

Copy and paste alphanumeric code created in the user setting of Biblio-retratos. This is only done once.