copying part of a tree that is on FTM

I have a large tree on FTM. I would like to leave that tree intact, but make several new trees from it. That is, I want to copy a part of the existing tree making it a separate entity. I have read the Companion Guide chapter on managing trees, but I cannot find anything that addresses this. It is my understanding that it can be done.

Can you tell me where to find the instructions on how to do this? Thank you.