I like to add any fuel additives before I fill up. I have two gas stations within a block of my house, so it's not a big deal to dump it in the tank in the garage then just ride there. Depending on your situation, you may want to take the bottle of additive with you to the gas station.
If you dump the additive in before you fill the tank, the act of filling it up will inherently mix it up well. You won't have to worry about the product forming a slick on top of the gas or just sinking to the bottom (depending on the exact chemical makeup of it), and you don't have to try to shake it around (by tipping the bike side to side or whatever) to mix it up.
If you can, try to find
ethanol-free premium gas for winter storage. Ethanol attracts water and can attack soft parts in an older fuel system. Gas will degrade over time, which the higher octane premium helps mitigate.
If you're feeling adventurous,
here are the details on the fuel additive I've used in every tank for over two years.
It's definitely not necessary, but a
GreaseNinja will simplify lubing your chain (which you should be doing every few tanks of gas).
You don't need to remove the battery from the bike, but doing so will ensure there are no parasitic drains wasting electricity. As others said, it's mostly a temperature issue.