TwistedTwigleg What do you mean? Like making the .pck files? There should be implementation details on how .pck files are made in the Godot source, though right off I do not know where in the source to look.
bitshift-r Here's the code for the engine implementation: pck_packer.cpp There is a file header with version numbers, then a table of file entries (paths, offsets, sizes, md5 hashes), followed by the actual file data.