Three procedures that sound the same
The government appointment site lists “asignación de NIE” (the number only), “toma de huellas” to issue your Foreigner Identity Card (TIE), and “recogida de la tarjeta”. Each has its own calendar and its own level of demand, and booking the wrong one costs you the day.
Fingerprinting comes after a favourable decision on your residence or stay. Without that decision there is nothing to issue, and the counter will tell you so.
When you can book
From the moment you are notified of the approval. The notice usually gives you one month to apply for the card: that is the deadline to apply, not a promise of a free slot. If the calendar is closed, keep dated screenshots of your attempts.
Renewals follow the same order: first the renewal decision, then the appointment to make the new card.
Step by step
Go to the government appointment website, choose your province and, under procedures, “Toma de huellas (expedición de tarjeta) y renovación de tarjeta de larga duración”. Enter your NIE or passport and your name exactly as on the document, with accents and full surnames.
If there are slots you will see offices, days and times. When you book you get a code by text or email and a confirmation: keep it, it is what they ask for at the door.
Why there are no slots
Offices release their calendars in batches, and each batch is gone within minutes. Between batches there can be days of empty screens. That is why watching continuously, and choosing every office in your province rather than the nearest one, works.
General information, not legal advice. For questions about your own case, check the decision you were sent or ask a professional.