If your map is a Disk Image File (all the ones that I've had for Garmin devices have been) then simply copy and paste it into the Garmin file on the watch. If it's named the same as a DIF file already in the folder (e.g. "gmapsupp") then add a 1 to the end of the name so that you don't lose the map already on the watch.
Edit: To display the map go to settings (by holding down the left middle button), activity and apps, hike (or wherever you want to view the map), hike settings, (scroll to) map, configure maps, then scroll to the map you want to display and select it. It's not as complicated as it sounds.