My process was:
1. Make appointment at Migraciones
2. 4 days before Migraciones go to interpol, fill out the paperwork, teeth examination, fingerprinting, etc. Be there as early as possible. Q's are long.
3. Pick up the Interpol results and go to Migraciones for your appointment. Hand in all paperwork. Make sure you have the 'permission to sign documents sorted out beforehand!
4. Wait 2 weeks to get Migraciones results (email). Migraciones gives you an application statement with a number. You can use the number on their website to track the application process/progress.
5. Pick up paperwork + carnet card.
Bring lots of copies of passport and photocopy all the forms for your own admin.
If you apply for a work visa, do not forget the pay the quarterly fee (US$ 20) and consult migraciones in time (2 months before your 1 year expires).
Remember that if they find a single mistake in the forms/process/etc you'll have to go through the whole thing again. Your Interpol results are valid for a few months.