@Alvaro *****
I got 60 days extension. Just go to immigration and talk to the volunteers. They will give all the forms that you need to fill out... mostly you have to write your name, date and sign. The other document that you need is the TM30 if you are staying in a friends house, otherwise, you just need the address from your hotel (not sure)....
Bring your passport (copy of the pages, there is a copy place just behind immigration) and 1 photo
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Once you have everything, they will ask to go inside and wait until your number is called.