File the KIT within ~8 days

The KIT (kit postale, kit permesso) is the application that asks Italy to convert your national visa D into a permesso di soggiorno (the plastic residence-permit card); the same form is later used for renewal. By community experience you should send it within about 8 days of entering Italy, and the deadline is treated as hard: miss it and the permit application can be refused. Start on day one. Verify the current rule on questure.poliziadistato.it and portaleimmigrazione.it.

You collect the KIT envelope at a Poste Italiane office (yellow logo), usually at the Sportello Amico window - say you need a "kit permesso". By community experience envelopes are limited in stock and you may have to try several offices; they are handed out one per person and you must come in person. You need one KIT per family member from roughly 14 years old; younger children are written into a parent KIT. Verify on poste.it.

Bring the originals to Poste Italiane; each applicant hands in their own envelope in person. The operator checks the passport against the form, takes the fees (per the community ~30 EUR postal plus ~40 or ~80 EUR for the permit depending on the requested term) and issues the ricevuta. The ricevuta is critical: it proves your stay is legal while you wait for the permesso, and the permit term is counted from this filing, not from the plastic card. Guard it. With it you get a sheet listing the Questura address, date, and time for fingerprinting - attendance is mandatory.