To everybody looking for the new decryptor file, you can find it here:
New GUI Decryptor
Link dead, can't find the file. Unless you've found the file, you should probably use the original post instead. (If you have found it, send me a message so I can add the link!)
It's self-explanatory, but they didn't call this in-depth for nothing.
Also, be sure to make a backup first. I'm not responsible if you lose your save data.
Step 1:
Click the three dots next to the field that says Input File.
Step 2:
Find your game.sii file, and press Open.
If you want the decrypted file to be as its own file, you can repeat Steps 1 and 2 with "Output File"...just make sure you rename it to something other than game.sii if you decide to do that.
Step 3:
Click "Start processing".
Once it's done, you can resume with the guide from after the file has been decrypted. Happy trucking!