I think that it would be a good idea to create a new configuration file /etc/X11/Xsession.d/15x11-synaptics because creating a new file is as easy as editing an old one. In both cases you need to be root. I would use the editor leafpad by issuing
Code:
sudo leafpad /etc/X11/Xsession.d/15x11-synaptics
in a terminal.

After you have created or edited the configuration file you must restart you Xserver in order that the new configuration will be used. In most cases it will be sufficient to hit the key combination Ctrl-Alt-Backspace. But do not forget to save your work before you restart the Xserver.