You have the nib, the feed, and the feed housing. Sometimes the whole assembly is referred to as a nib (even though the nib is actually the metal tip by itself).
You will notice there are (at least should be) some threads on the rear of the feed housing. You need a tap in this size and a corresponding sized drill bit to tap the section to accept it.
The section is the front (removable) part of the pen body that holds the feed housing, you need to make this in a size amenable to your feed housing and pen body (it is made to screw in to the rest of the pen body).
Once you have made the section, if you are certain you won't ever need to remove the feed housing from it, you can glue it in instead of tapping and screwing it in. Usually not done that way but it will work, I managed to destroy the threads on a feed housing and did it this way to avoid scrapping the nib assembly and it worked out fine (so far). The taps tend to be odd sized and a little hard to find and expensive.
