Uploading OSM on Garmin with Mac


Just registered, and hoping to get some help.
I have a Garmin 705, and am using a Mac. And I would like to get the OSM map for Victoria on my Garmin. How do I do that? I have the 3 .img files, but can’t find any explanation on how to combine them and load the map onto the Garmin.
Any help very much appreciated.