Do this, do that, do this, no dont do this, do that.
Wow...this is like asking what oil to use.
Most of us dont store these for the winter, but in 'salt' country, I can understand.
Unheated garage is fine. Air-cooled metal motorcycles (with nothing to freeze, other than battery electrolyte) do NOT need heated storage, no matter what anyone else, here or anywhere, says. Period. In fact, cold storage reduces oxidation. If there are small air leaks and drafts in the garage, like under the door, its OK as long as its venting air, and hopefully not rain or snow coming in. Just be sure to keep the battery on a maintainer.
Change the oil, all three places: Crankcase, gearbox, and final drive. This will remove any and all moisture in those oils. Add some fuel stabilizer like Seafoam or Startron to the gas in the tank
and run the engine long enough to bring the stabilizer into the FI system..
If possible ride the bike for 5 minutes to circulate everything.
Fill the gas tank to maximum, right up to the top. This displaces air in the tank and prevents moisture in the gasoline/ethanol from causing rust in the tank. It also reduces surface area for moisture in the atmosphere from being absorbed by the ethanol. If you can find pure gasoline, with zero ethanol, I highly recommend you fill the tank with that.
As an alternative, you can completely drain the tank...I prefer not to.
Put the center stand down, lifting the rear tire off the ground. Pull the rubber mats out of the sidecar and trunk and store them separately. Empty the sidecar trunk and prop the lid open an inch or two with a block of wood. Its OK to leave the tools and airpump in there, but you dont want anything in the trunk that can leak or freeze, like water, flashlight batteries, sunscreen, etc.
Hook up your battery tender (maintainer) and make sure it is charging.
Attach a tarp, cover, or plastic sheet loosely over the bike, to keep the accumulating dust to a minimum.
Close the garage door and go have a beer! (gobium inspired!)
Check on the maintainer once a week or so, or at LEAST once a month. Do not start the bike all winter unless you plan on riding it for 10 minutes or more, but its not a bad idea to slowly stroke the kick starter a few times with ignition off during storage to move the pistons around.
When you first ride the bike in March or April, let us know, we want to hear about the ear-to-ear grin on your face when she fires right up, ready to go!