My advice would echo a lot of others.
I put a foil-backed picnic rug on the floor of my tent, and either a good thick SIM lying on a 2nd blanket on top, or put my SIM on a campbed. Good 4 season sleeping bag. Extra rug to put over me initially, or when I wake up chilled.
Always wear a wolly hat in bed, I usually have a hoodie of some sort as my outermost layer and can put that hood up too.
I also keep a pair of thick fluffy hiking socks in my sleeping bag - they get dried out every morning, but they are not put on my feet until I am getting into the bag for sleep. (I put them around my hot water bottle in the bag while I go to the wash block for the last minute visit to warm them up).
I always bring a hot water bottle and fill that from the burco as I am closing up the marquee (on Cub camp) or my own small kettle/previously filled flask if on family camp. (Cubs never see it, and the other Leaders just laugh and write it off as me "being the girl" - but also know that I'm the one most likely to be woken during the night, being the "mammy" figure!!).
In the early evening, so around dinner time (depending whether that is early or later), before it starts to get chilled, I change. I put on a thermal base layer top and bottoms (merino), and my winter lined hiking trousers, and at least 2 layers on top (a long sleeved top and a hoodie). Tuck the layers into each other so there is no air getting straight to your skin. If a cool night, I will put the wolly hat on while outside. And I will make sure I am wearing socks at that stage (I may have been in flipflops earlier so sockless - but socks in the evening are NOT my sleeping socks).
When going to bed, if I change anything, it is only the outer layer and not the thermals. Change top and put night layer on. Only when top is again covered will I take off bottom layer (trousers) and change that - opposite way around is fine, but its about keeping as much warm air next to your skin so the least amount of time it can escape, the better. Have everything that you are putting on ready beside you before taking off the discarding items. I will often sleep in my hiking trousers, or else I put on a pair of comfy PJs - and often with between 2 and 4 layers on top, depending on outside temps and conditions and how long I have been sitting outside. (And possibly on the likelihood of having to get up in the night, so whether leaving harder wearing things on is better - hammering in pegs at 4am, or dealing with a puking 9 year old, is better in winter trousers than flappy PJs).
Good luck.