You must be in the US, Canada, UK, or the EU
You need to have made a purchase on Steam between November 14, 2021 and November 14, 2022
Your account must be in good standing
You cannot have a limited account
TLDR; Register here and then watch the stream while logged in here or here on Dec 8th.
Pro tip: set a reminder while registering.
For in-depth info follow this discussion thread https://www.steamgifts.com/discussion/48bzc/free-steam-sticker-and-chance-to-win-steam-deck-on-dec-8th
Edited