You’re not sure how to attach the headrests, or the trim, or both?
I can only remember the two posts on each headrest which screw into the back of the seat … no recollection on how that trim attaches. Having seen James’ post after this, I should reiterate - I’ve only seen the early headrests, the link below are later and while I may be mis-remembering, I do think the later ones differ (and the link’s great if that’s what you have).