Easiest way, find somewhere to start (like an existing Arrow). Study it in the editor.
Create a new Item ID (safest way is to use a 7-digit item), with the same exact settings, save and update your files (server/client). Test and run your server/client. if your new item works as intended, then you can simply open it in editor again, adjust its name/values and you are done!
Dont forget to add/edit the applicable strings.
ObjEdit is to modify objects properties from within the game as a GM.