When in petitboot exit to shell. You're now in a basic linux commandline. You need to navigate to the directory containing GetKey, which is on a usb stick. Two commands you need are ls (to list the files in the current directory) and cd (to change the current directory), read up on them if unfamiliar.
Some files are listed, one of which should be the usb stick (likely usb names are something like sda1 sdb1 etc). Try them all by going in and listing them until you find your stick. Then you just put a disc in and do this:
./GetKey 3Dump.bin >output.txt
This dumps the disc key, disc id and pic and puts it into file output.txt in the same directory as getkey. You could also grab the full pic if you'd like by doing this:
./GetKey 3Dump.bin big.pic >output.txt
output.txt is the same as before, what's new is big.pic, which is the full pic. It's not required by redump, but it doesn't take much to store it, and who knows it may be useful later.
But before you do any of that, you need 3Dump.bin and put it in same directory as GetKey (easiest is both in root of usb stick), get it by running a pkg as described in the guide.