1- Draw the pen you want to build. It doesnt have to be a cad drawing to work.
2- Decide on dimensions, lengths, diameters inside and out. (ID and OD)
3- At some point choose a brand of Fountain Pen nib and feed you want to use.
Figure out where the threads have to go and what ID and OD you will need for them
Figure out how to hold it while you make it.
Drawing the pen will help you focus on the relationships between the parts. If you make one bigger does it make something else bigger or? For metric you can subtract the Pitch from the diameter to get the hole size you need and for the Tenon that will be threaded. Subtract about .2mm from the diameter.
Alumilite blanks, aluminum, Ebonite are good materials to make a kitless pen.