For me, serious winterization comes before the M235i goes down to the Swiss Alps for its first ski outing of the season. By that time, it's already got it's 7.5Jx17 square wheels on carrying Pirelli S3 winters anyway, just in terms of cold and wet.
I have a 5L BMW Winter screenwash Concentrate [83.12.2.409.032] which is more cost-effective than the 1L versions, and start to up the concentration as the autumn progresses. I'll then also carry
- 1L Bottle of made-up screenwash top-up
- External Windscreen foil cover
- BMW Extending windscreen scraper/snow brush [83.19.0.412.029]
- Small scraper I stole from my hire car
- Spikes Spider EASY Sport Snowchains
- Rechargeable lighter torch
- Old but thin Gloves (for snowchain fitting)
- Carpet Square (to kneel on)
- Emergency food/drink Pack
And if it's the long drive down to the mountains, I also make sure I have boots handy to change into if I have to go out in the snow/slush. The windscreen cover is one of those dual summer/winter things with thick foil. If we're parking outdoors then I make sure to cover the wipers and air intakes at the top of the bonnet, so they don't get clogged by snow. For the Alps, the screenwash gets refreshed to almost pure, so operates down to -30⁰C (-22⁰F) - probably overkill as the worst ambient I've been in in the mountains was 'only' -24⁰C, at altitude.
I wish I'd gone for the more expensive front-fitting Spikes Spider chains earlier as they fit a far wider range of wheel sizes than conventional ones. I wouldn't now have three sets of chains in my garage from different cars, all of which the Spikes would have fitted with some adjustment of the links. On top of that, they genuinely only take a couple of minutes to fit. Which is important if you're the guy holding up all the traffic.
And I practice engaging DSC=OFF [TRACTION] DTC=ON for if I hit snow/slush. First time, I had to stop to read the manual, which was bad preparation on my part. Confusing terminology from BMW always makes this more complicated to understand than it really is. Basically, treat the car-with-wiggly-lines-OFF button as if it had a snowflake on it i.e. a 'Snow/Slush' button.