These pen blanks are designed for the 8mm metallo pen kit available from Penn State Industries in chrome or gold finishes. The "ARTG" series of blanks are "Almost Ready To Go" which means that the outside diameters of the blanks are very slightly oversized relative to the correct final size for this pen kit to attempt to compensate for 3D printer process variations. You don't necessarily need a lathe to prepare your blanks prior to assembling your kit using these blanks.
There is however some DIY work you must do to your blanks prior to final pen assembly. These steps are basically gluing the brass tube(s) in the pen kit into your blank(s), sanding the ends of the blanks down to the ends of the brass tubes, optionally sanding the outside surface of the blank by hand (or in a lathe if available), and surface finishing your blank with a sealant or other finish as desired. If you choose one of the polished material options the final outside diameter size will be reduced to approximately the correct size by the polishing process (polished white strong & flexible, colored strong and flexible except for black, and polished alumide). If you choose a non-polished material option you may want to sand the outside of the blank by hand or on a lathe to get the best fit to match your pen hardware. Note that sanding or machining any of the colored strong and flexible plastics may completely remove the thin color layer at the surface of the printed and dyed material.
I prefer selecting alumide or polished alumide for this type of blank. Not only is the color relatively uniform throughout the material but it is porous enough so that you may attach the blank to the brass tube in the kit by "gluing through the blank" with thin CA (Cyanoacrylate) glue. This is accomplished by prepositioning the brass tube snugly and centered into the blank and then saturating the blank with thin CA glue in a disposable container. The alumide is porous enough to allow the CA glue to soak through all the way to the brass tube. Quickly blot off excess CA glue from the blank and allow it to dry on a wire or dowel rod completely before final pen assembly. Follow all manufacturer safety precautions relating to the use of CA glue including skin, eye, and breathing protection.
Using other less porous materials may require additional care when gluing the brass tubes into the blanks since glues or epoxies can inadvertently stain the surface or the blanks if they are not protected. I have had success protecting the surfaces of blanks using removable painter's tape prior to gluing.
Yet another option is to use no glue and have the pen hardware hold the blank securely in place when it is assembled in the pen press. To do this it helps to have a very snug fit of the brass tube in the blank. There is no right or wrong way to assemble pen kits. Experiment with different techniques and methods and use what works best for you.
For more flexibility many designs in the ARTG series are also offered in the "L series" lathe option. These lathe-able designs have identical or very similar patterns to their ARTG equivalents but with larger outside diameters to allow the experienced pen maker more control over final pen blank dimensions.
For final pen assembly it helps to use a dedicated pen press to achieve good results. Alternately you could use a large C-clamp with wood or plastic covering the clamping faces but you need to be extremely careful to not crush or damage your pen mechanism.