Looks like I'll have to begin again. I'll keep the main yaml files, which have already been edited. Redownload the patch and the firmware (it was hard to find the firmware my Kobo's running 4.30.18838 to download, everywhere I looked was offering the 4.31 firmware).
I'll put the zipped firmware into the src folder.
What determines whether I need to install this firmware onto the reader at this point? It already has it - it came with this firmware already installed.*
*Once I know the answer to this question, I'll proceed as follows, unless someone tells me I shouldn't.
Replace the four yaml files with the ones I've edited.
Run the .bat file. Then put the KoboRoot tgz into the .kobo folder of the Libra.
And hope it works!
If this is the correct proceedure, please let me know and I'll try again - without making such a pig's ear of it!
