Attunement is a rule that applies specifically to magic items.  Rules for attuning a magic item can be found in Chapter 7 of the DMG.  Normally, a magic item is attuned to the user of the item and in order to attune the item, the user essentially meditates on the item for the length of a short rest (after having already discerned the magical properties of said item during a previous short rest or through other means).  

This item, in particular, requires DM intervention to use correctly because we don't have rules for attuning magic items to a location or a plane.  Given that the attuned user needs to meditate on the item for an hour, I would say that the *plane shift* tuning fork needs to have originated from the plane to which it is attuned.  And, given that the actual nature of the device is that it is a tuning fork (a forked metal rod) attuned to a specific plane, I would say this type of attunement happens once at creation (perhaps the fork is attuned to a specific frequency associated with the plane), and can't be changed.

In any case, the PC in your example cannot create this rod himself for the purposes of banishing creatures to oblivion.