i can imagine that it's not nice - i'm not particularly senior in my field and only ever had to give a dressing down to one of my subordinates before and even then i was a bit of a push over about it. it's never nice upsetting someone, but as i potentially have to give bad news fairly regularly in other, more serious contexts, i have been given some training on how to let people down gently. advice is to ask them first if they know why they've been called to your office, fire a couple of warning shots, and be clear and concise rather than beating about the bush.
i know you mention that they have had previous appraisals before but if it were me i'd also want clear constructive criticism on how i may improve or where/how i might go about getting further experience in the field in case its something they wish to work at and return to.
sure i'd be upset regardless, but i'd be less upset if i knew how i could improve rather than being clueless. i'd imagine that with the shock of this a little common sense would go out the window and it wouldn't be clear to refer to previous appraisals and the like.
best of luck
