On sight of your letter from the Ministry of Tourism, the police will issue their travel letter.
The police office is on Trunojoyo Road, east of the ASEAN Offices. You can only enter the police compound on foot using its east entrance, and you need to enter the building itself on its north side.
You need your passport, a photocopy of your passport, the Ministry of Tourism letter, a couple of passport photos, your "story" and your Carnet. Topped off with a Rp 50,000 note.
In return, you are given a travel letter, with your vehicle's registration number, your photograph and an expiry date.
Now present these letters to your custom broker, and he'll interact with the Customs. See later for my recommendation for a decent customs broker in Jakarta.