There are a few ways to winterize a tent. The basic idea is that you are trying to minimize heat loss as much as possible.
To that end, the first thing you want to do is choose as small a tent as practical. Larger tents require more heat to warm them.
You also want to make sure that your tent is up to the climate you’ll be camping in. If it’s going to snow, you need a four-season tent.
The next thing you need to know is that tarp is your friend! Stick tarp to the top and bottom of your tent to keep the heat in. You can also use Mylar blankets or any waterproof blanket. Make sure to tape or tie the tarp in place, so it doesn’t blow away.
When it comes to the inside of your tent, put something insulating on the ground. This could be a heat retention sleeping mat or blankets.