In the context of this tool, the PS4 Account ID is the 16 character hex string (example: 01ABB2CDR4EFT5HF) that you see in the filepath when you copy saves from your console to your USB drive. The Playstation Network Username is your PSN username which cannot be used for resigning saves. You can go to your Profile settings page to set a PSN username and the tool will attempt to get your PS4 Account ID that can be used for saves and save it to your profile so you only have to do it once.
First make sure you have an PS4 Save Account ID set in your profile settings. Not your PSN account name, but the 16 digit PS4 account ID you see in the filepath when you copy a save file from your console to the USB. Then, open a ZIP file smaller than 200 MBs containing your save file and save key: SAVEFILE and SAVEFILE.bin. It will resign the save file to your account. You can provide multiple file pairs in the same ZIP file. Including more files means the process can take longer than usual.
Open a ZIP file smaller than 200 MBs containing your save file and save key: SAVEFILE and SAVEFILE.bin.
First, open a ZIP file smaller than 200 MBs containing the target save to inject into, so this is your save file and save key from your USB: SAVEFILE and SAVEFILE.bin. Then, select a ZIP file containing the previously extracted/modified save data to inject. You can inject 1 file or all files into the save file at the same time. The file injection tool does not support injecting new directories into save files that don't exist in the save already. But it does support injecting new files in the root save directory. This feature will come later.
You can resign pre-existing saves using your PS4 Save Account ID. The tool does not currently support entering your PSN username but support is on the roadmap.