If you search the forum for Winter storage, there are several posts addressing it. I didn't change the oil this year as I only had 1,000 miles on since the prior change in June.
Otherwise, wash, wax, top off tank and add Stabil. Plastic sheet on garage floor to block moisture with some planks positioned to where the 50 psi tires will rest.
I bought some dessicant gel packs (like the ones that come with new shoes), and I toss a couple in both footwells to keep the interior dry. Plug in the battery tender and gently place the fitted cover on top.
Then I put small plastic bags over the exhaust tips and rubber-band them secure (one of the storage posts suggested steel wool) to keep small animals from crawling inside. The final step is to put shallow plastic bowls at the corners and "amidship" with a handful of mothballs in each to ward off mice and such.
When I purchased my cover last year from Amazon, they actually shipped two but only charged me for one. I called about returning one, and they said their records showed they only shipped one.
So I have a never used Coverking indoor cover, green & black.
Coverking Custom Car Cover for Porsche Boxster - Satin Stretch Fabric, 2-Tone Black/Green : Amazon.com : Automotive
Willing to sell for fair price.
DBear