Download macOS

1. Get FetchMacOS and place it in a known folder.

2. Open Command Prompt and use cd to go to the known folder.

3. Run fetch-macos.exe -p PRODUCTID -o SharedSupport where PRODUCTID is the macOS package you want to download.

Edit Downloaded Files

1. Rename InstallESDDmg.pkg inside the SharedSupport folder to InstallESD.dmg

2. Edit InstallInfo.plist using WordPad (or another text editor) and remove the chunklistURL and chunklistid keys for InstallESD as well as rename InstallESDDmg.pkg to InstallESD.dmg (example here)

Write BaseSystem to USB

1. Format the USB drive using Boot Disk Utility. Select DL and Save after DL for Clover Bootloader Source, Clover for Format Options, and a 200MB Boot Partition Size for Multi Partitioning

2. Extract 4.hfs from \SharedSupport\BaseSystem.dmg --> save to it's local folder

3. Restore 4.hfs to the USB's second partition ---> 2nd partition becomes bootable "OS X Base System"

Resize BaseStstem USB Partition

Open Paragon Partition Manager Free and resize the second partition of the USB drive to the full size allowed. Don't forget to apply the changes at the end.

Copy Additional Data

1. Open TransMac and navigate to the HFS+ Volume/Install macOS High Sierra.app/Contents folder. Right click anywhere in the empty space on the RHS pane and select Copy Here.

2. In the next stage, select the SharedSupport folder we created earlier as the Files and Folders to be copied to the Mac Volume.