Session can be started from the $HOME/.xsession in last command line as exec nscde, or exec /opt/NsCDE/bin/nscde or ssh-agent nscde or with gpg-agent, lxsession or whatever.

If supported by the X Display Manager which is in use, an xsession file /opt/NsCDE/share/doc/examples/xsession-integration/nscde.desktop can be put in /usr/share/xsessions (or in whatever place your system and your X Display Manager reads this files) and then selected from the manager's menu or similar selector. See the rest of the X Session Manager integration examples are in directory /opt/NsCDE/share/doc/examples/ for MATE, KDE, LXDE and similar DE integrations and play with this if you like.