Use this website:
Free worldwide routable Garmin maps from OpenStreetMapPick France from the dropdown list at the top, then click the "Download map now" link on the left. That will give you a choice of files to download, for copying straight to a card you want the "osm_routable_gmapsupp.zip". Then just unzip that, and copy the gmapsupp.img file to a folder on the card named Garmin.
Note you can also use that website to make a map for a custom area, by using the manual tile selection option. Though if you do that, you have to fill in your email address, and wait for it to generate your map, which might take a while. So its quickest/easiest just to pick a predefined country.
