How do you best help trees that have been damaged by severe freezing?
Not a lot to be done, damage done is , well , done. Best to just make sure they are fed and watered well, and in the fall you could trim any remaining dead wood though that's not necessary.
It's probably a bit early to determine if a tree will survive, let spring get here.

Trees are very self sufficient. Probably better than 80% of so called "tree care" is actually human care.