Transitioning seedlings to outside

I’ve transferred several grown from seed seedlings. Spruce, mountain pine, and tons of boxwood maples. They all die in about 1 month of being outside... any tips or pointers for making this transition successful ?

I have trident maple, Chinese juniper and Japanese yew seeds that are currently in cold stratification and I don’t wanna lose them.

Sow them outside not inside then you would not have the problem,