Same as what has already been said, keep them well fed, watered, warm and handled.
IMHO it’s the wet that gets to them more than the cold, try and keep them covered when it’s wet.
A meat diet is best. But if you do go down the dry route, supplement it with plenty of meat and the occasional egg.
I bedded mine in either straw or shredded paper. Just gave them a big pile in the sleep box. If they were too warm they’d kick it out. If dragging shavings in, they needed a bit more bedding.
Sorry for the long reply. This worked for me, others might think it’s a load of shite!