Helping a clueless newbie - can't install maps on my Nuvi 255w

First off, I hope this is the right section. I tried following lots of tutorials, but they didn’t help me.

I have a Nuvi 255w with NO maps on it.
I downloaded the City Navigator North America NT 2013.30 original untouched GMAP installation (FID/MapID = 2775) and then extracted its content to a local folder.

From now on, I have no idea on what to do next. I’m pretty sure this is already covered here on the forums, but I really don’t know which tutorial I should follow (I tried a lot, I swear, maybe it’s because I’m stupid or Brazilian – or both lol).

So if you guys could give me any directions I would really appreciate it. You don’t have to write a new guide for me here, just point which I have to follow, what programs I should have/download and I’ll figure out the rest.

Thanks a lot for your help.

Best regards