From IAP-Wiki

Load Mandrel

Install the bushings and blanks and bushings on the blank in this order.  The bushings should be a close fit on the brass tubes.  A single tube pen only has two bushings.  In this case you can put any spare bushings to make up the remainder of the mandrel space.  

1. Nib bushing.
   
2. Nib blank with center mark pointing to left.
   
3. Center bushing(s).
   
4. Cap blank with center mark pointing right.
   
5. Cap bushing.
   
6. Mandrel nut.  Snug up but don't over tighten.

Install mandrel on lathe

Insert the mandrel taper into the lathe headstock.  Bring up the tail stock and lock down the tail stock.  Extend the tail stock live center point into the hole in the end of the mandrel.  You should have a 60 degree tail stock live center.  Snug up the tail stock to the mandrel.  The live center should spin with the mandrel.