LOL! How?
This is not a pak96 problem/question!
The winter cames by itself. If you have the right options.
What are the right options?
BTW, the starting map (when the game starts) can let it snow...
It depends on the different climate levels you've set up in the climate window at the begining of the game. If I remember correctly, there're a winter snowline and a summer snowline parameters. They're also related with map heights. If the map terrain is plain, you'll have to set up a low value for winter snowline.
So...
if the summer snowline is 24, the winter should be likely 18? Right...
No.
Summer snowline = the height at which it will ALWAYS snow
Winter snowline = the height below which it will NEVER snow
Try summer=8 and winter=1, then adjust from there how you like