Cutting parts off of the "trees" they come on is a
skill for model railroading. If you try and twist the part
will damage it. Place the tree flat on your work desk and
cut it off with a sharp XACTO knife or a set of
rail nips. Lightly sand any excess material
still attached to the part.
I will try and take some pics so that I can better demonstrate
what I mean.
For now, remember don't twist your parts off.