If it’s one night, and around -10c, I use my light weight -5c bag and mat with an electric vest and electric socks on the lowest setting. They usually work till the morning. I can fit pretty much everything in a 30l bag.
if it’s multi night or like -20c, bring some much thicker stuff. Down although packs away better and lighter, it tends to stop insulating when it gets wet. It’s very likely the condensation will get almost everything in your tent wet. 50l bag gets me through a couple of nights.
your head is the best at losing heat. I like to sleep in a fleece hat (sometimes two), down puffy hood from bag or coat.
if you’re still having a hard time getting warm, boiling water and throwing it in a Nalgene and in your bag between your legs does wonders and will give you a couple hours of warmth.
If you’re using a jet boil, sleep with your fuel. Iso doesn’t really work if it’s cold.
throw wet stuff into your sleeping bag with you. I generally sleep in my boot liners just so I don’t have to put them on frozen.
if possible, use sticks for your tent/vestibule anchors. The stakes are just too small for powdery snow. You spend more time fixing them and packing down snow than just cutting some small branches.
Don’t depend on dry frozen meals as your entire nutrition. Bring other dry stuff too. My go to is oatmeal, soup powders, hot chocolate. Nuts and granola bars have good energy a fibers too. I always have beef jerky.
find a spot with wind cover. Or dig yourself a wall or something like that.
idk, the first time is nerve racking, but three or four times in and you get used to it and know the deal. Good luck and enjoy. It’ll def be memorable.