Been in a tent with temps as low as -15, -20 with a small stove. few things I'd recommend:
- get off the ground with some type of cot (like alps ready lite cot)
- use a good sleeping pad (high R value) like Therma rest X Therm
- sleeping bag liner (Like the Sea to Summit Reactor)
- have dedicated clothes to sleep in & leave them in the bag during the day. (don't wear the same socks you wore during the day obviously)
- good base layers (merino, etc.), definitely have a good warm toboggan
- wear a puffy jacket if needed
- I never wanted to risk a leak by putting hot water so put 1 of those hot hands in the bottom of the bag & 1 near your torso. (this may be too much heat depending on your tolerance)
Some combination of these should be adequate. The first night is always the worst.
If you're with other people have a rotation planned for who gets up first, 2nd, etc.
- pre stage a fire starter, kindling & wood for 2-3 times of getting up to stoke the fire if necessary. That way you're not having to turn on lights, breaking sticks, tossing wood around & making a big racket.
- keep a pee bottle close so you don't have to go outside & let cold air flood in the tent
I also keep my phone, Inreach, power bank, etc. inside the bag
Also be sure to keep a window (if your tent has them) slightly unzipped to allow a little air flow. This will greatly reduce condensation.