It's best to remove the parts from the trees or sprue by cutting, rather than twisting them off. Do this by cutting the part off as close as possible to the surface of the part so there is no excess plastic on the part from the tree. Most modelers will use a hobby knife or sprue cutter which you can find at most hobby shops, but even a nail clipper will be better than twisting the part off and risking breakage, especially with thin or fragile parts.