The nib tip is split into tines, these should be aligned. If not align them gently.
The nib slides over the feed into the nib unit.
Gently push the nib down until about 3 mms on the nib tip (tines) show above the feed.
The nib has a natural position, so do not force it, just push into position gently.
On the Churchman's Prescriptor the nib may require turning so that the openings in the feed are equal both sides. The Captain's Commission, has a guide slot inside the nib unit, make sure that the feed engages with this slot before pushing gently into position.
If you get stuck, just telephone us on 01895 630361. Pens can still be serviced by us, so if you do not feel confident, just return the pen. The above covers servicing only, repairs are best carried out in the MrPen workshop.