If you have the map now in Mapsource you have as Neil says done the hard part!
In Mapsource select the Map tool (in dropdown Tools or with Icon), then you can select one or more tiles from the map, generate the gmapsupp.img file and transfer it to the gps. The size of the selected tiles can be seen to the left in Mapsource. I noticed that some tiles were bigger than the memory in your gps so maybe some part will not fit in it.
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