To make your own files, you either need to digitally model the pieces from scratch, or find 3d models that you can modify and use (such as from a video game). Both of these methods require 3d modeling software and experience to accomplish, and the skills needed are not really teachable in a single tutorial (or two or three). Creating a 3d model from scratch to use for pepakura can take a long time even for experienced 3d modelers. For less well-known or popular characters, it's usually fastest to commission a 3d modeler to make the files for you, rather than wait for someone to make them on their own and release them.