Firstly, if you're really unlucky or misguided, you may have bought a unit without the power cable. Mine came with the cable chopped up to fit a Bosch power socket similar to a cigarette lighter socket but not quite, these are common fitment on BMW motorcycles for some reason. This was a problem because the wires in the power lead are aluminium (!) and the strands would work harden and break off under the screws over the course of a day. As you can imagine, stripping back the wires and reattaching them to the connector every morning became quite tiring. If you need a cable then I found mine on ebay under the title "GARMIN StreetPilot 2610,2620,2720,2820 GPS DATA BMW Amplifier Harness Kit Cable" but I think it also has the BMW part number "71 60 7 686 670". It also bears the code "320-00171-53" if that helps anyone..
With that you'll need to know which is +12v and which is negative, the Garmin cable is traditionally coloured with the red wire is live and the black wire as ground but a kind soul over on the advrider forum has made a lovely diagram:
With a bit of wire stripping, twisting, soldering and heatshrinking done it was time to wire the Nav into a battery and hope that the people on advrider weren't lying to me..