Transport OSM selfmade map for Garmin into GPS

Question:

Just bought my Dakota 20, GPS functions: found myself on the right spot!
But, the world-map is not very precise though.

My first bubble: I generated the .zip map, to be used for the GPS, thanks to Lambertus,
but what do I do: put, copy, transport the unzipped file into the GPS or the zipped file, from which I cannot change its name. Thus I hope to copy the unzipped map, otherwise it is troublesome how to find the right map in the Dakota.

Thanks for a help.

Hint: There’s a subforum for Garmin-Stuff

o.k., I’ll post my question there.