Tips:
1. Warm them up slowly
2. warm them all the way through before returning them to a WARM nest.
Ways to warm them
1. heating blanket set on low
2. warm water bags
3 heating rice bags.
etc.
My preferred method is to place them on the edge of a warm litter of older kits. This warms them up well and I'll often find them later in the middle of the heap. If I don't have an older litter I will bring them in and put them in a warm rice bag with towels from the dryer all around them.