I’ve tried to figure out this OSM map thing. Going to Chile in 2 days. I can’t find any documentation that is step by step. This website makes no sense: http://garmin.openstreetmap.nl/ – how are you supposed to select anything? To me, the documentation is not user friendly and impossible to navigate. OSM directions need to be clear and easy for non programmers and IT geniuses.
Welcome! 
Please help us to help you: What is “this OSM map thing” and how is it related to your trip to Chile? What do you want to achieve?
Oh, and for getting better: which (URL) “documentation” did you view?
I wanted to download free Garmin GPS maps to Chile. This http://wiki.openstreetmap.org/wiki/Garmin.OpenStreetMap.nl:Manual is of no help.
Oh, wow, I never saw this page before… although I am not new to this topic.
Okay, I will try to help you getting a OSM-based map on your Garmin GPS. https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin is our general Garmin help and info page. And https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download lists some pre-made maps. The one you have tried to use ( http://garmin.openstreetmap.nl/ ) is probably the most popular one. Note that all of those are private individuals or groups (the core OSM just gathers the base data)
Okay, go to http://garmin.openstreetmap.nl/ . Skip the page until the country selection, click on south america (you may need to scroll to the right). Select Chile.
(Oh... Interesting. For me it then shows Cook and Pitcaim islands on the map below, which is obviously not Chile. This seems to be wrong. Do you experience the same? I will report this, if this also happens to other people. It only happens sometimes. After clicking on some other countries, resetting selecting it correctly selects Chile on the map. However, the download link seems to be right anyway (indicated by country.txt on the download page))
In the map below you now see some dark blue rectangles covering Chile (and more). That will be the map coverage. Now, just click the “Download map now!” link above the map. Read the description column of the table to determine what you want. Most easy is using “osm_generic_gmapsupp.zip”. Then you do not need any additional software on your PC.
Download, extract. Copy the the extracted gmapsupp.img into a folder named “Garmin” on your SD-card or device (if it has enough memory). This is described on top of https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download