Installation

conda install -p ~/.jupyter/ -c conda-forge jupyter_nbextensions
conda install -p ~/.jupyter/ -c conda-forge jupyter_contrib_nbextensions
jupyter contrib nbextension install --sys-prefix
jupyter nbextension install https://rawgit.com/jfbercher/small_nbextensions/master/toc2.zip  --user

Enable

jupyter nbextension
#Please supply at least one subcommand: disable, enable, install, list, uninstall

jupyter nbextension enable toc

#Enabling notebook extension toc...
#    - Validating: OK