How do I extract and recompress core.gz?

[NoIdea]

Hello,
I want to edit my core.gz to add custom files, remove some files, change my keymap, etc

So far  I have been trying to unzip core.gz , core.cpio and to repack them using file-roller for core.cpio and gzip for core.gz.
But the new core.gz is even with the highest compression level slightly bigger than the original one ( 5.7M vs 5.8M ) and when I try to boot from it it causes a kernel panic because it cant mount a root file system.
How must I archive and compress my custom core.cz?
And is it even possible to boot from a custom core.gz or does the kernel check with a checksum or something if it is loading the correct image?

[gerald_clark]

Instructions are in the remastering section of the wiki.