Without the plug (carbon steerer?) or a starnut you won't be able to adjust the headset properly so you will either have to improvise the adjustment or place a stack of spacers above the stem to let you use the plug or a temporary starnut. Once you determine the right height, remove the plug or drive the starnut down and cut the steerer to length.

BTW, I recommend leaving the steerer long enough to require a 5 or 10 mm spacer above the stem after the final length cut. That assures the stem is fully supported by the steerer (highly recommended for a carbon steerer) and gives you some height adjustability for future changes.