Not sure what exactly your issue is.
A Spice directive is nothing else than a text statement. What are you after?
I routinely add Spice directives in LTSpice schematics, for instance to add coupling coefficients between coils, or define initial conditions.
Just go to Edit/Spice directive, or just type 'S', the dialog pops up, enter the directive, place it on the schematic, done.
To edit one already placed on schematic, just right-click on it and the dialog pops-up with the directive you can edit.
Is there anything more to it?