Start with a short POS lead with a closed end terminal right on the Batt. and go right to a 10 amp quick blow fuse to protect the bike if it shorts out beyond that. I cover the section from the batt. to the fuse pretty good with Rescue Tape, Shrink tube, whatever. You don't want that to short out.
Now I go from that to a Short two terminal connector. Available at Wal Mart, CB Shops, radio shack, Whatever. It has a plug with one side insulated and another not. At the battery side the Red is insulated of course to prevent a short when nothing is plugged in. Many Off Season chargers have this set up already, So, if you have one, you will already have that Plug on your battery. I have seen them in Boating stares also. Looks like two prong trailer flat plug connector. The 6th Plug down on this page:
http://www.solarseller.com/low_voltage_dc_pumps__lvm__teel__accessories__plugs_and_extensions.htm
When doing electric on my bike I like to stick with boat supplies as they are made to much more stringent standards that I feel is needed on a motorcycle. Standards that pertain to Vibration and weather resistance.
NOW you have a short lead and a RED wire with an insulated female prong and a male bare prong right beside that (Black Ground wire)
I do it this way because I can buy may of these plugs and secure them to my heated gloves, Cig sockets, And aux Fan to run on engine at a parade, OR even a different length Cig Lighter socket, OR for long trips I even have one with 3 Cig Plugs that plugs into it to run XM, Radio, and Chargers or whatever.
Now, Decide how long you want your Cig Lighter socket, buy a socket at say Wal Mart in boat Dept. (I like the kind with a lid that springs closes so nothing gets in there when empty and shorts out.) Buy TWo what the heck. I have One that goes from the plug right into the Saddlebag to charge the cell phone or run XM. I even have a 12V AA batt. charger I run in there on trips. I also have a longer one that can go under the tank and up to the Pouch on the Windshield.
Plus, I can plug in my gloves, GPS, or my charger without taking the seat off. (Not a big deal on the Roady but it is on the Honda)
And if you want to get real wild, and not go over 10 amps total, You can even make a little harness that plugs into that
main plug and has say 2 or 3 or even 4 Same type plugs on it so you can have several things plugged in all at once, (And realize you don't have to run them all at once.) The fuse will correct you if you go overboard, Carry extras. I use the Tube type as they are less expensive for quick blow. Plus I use a very small dab of dielectric or bulb grease in all connections.
If I was home I would have posted pictures to go with the write up.
MC