Connect your gps to the computer with usb.
Select the parts of the map you want to transfer with the "select" tool in Mapsource.
Click the transfer to gps icon in Mapsource, select internal memory or sd card and there you go.
Remember that the existing map in the gps can be overwritten, backup before to be on the safe side.
And best of all: there are some great tutorials in this forum, check the sticky threads!