How to Offline Install macOS from DMG File

This method works with both installed MacOS and blank SSD drive when the recovery is loaded. I suppose that you are already have a DMG file with OS, you plan to install

Create new partition

1. Open Disk Utility, View, Show All Devices
2. Select your internal drive
3. Use Partition command, press + button to create new partition. The format of the partition must be MacOS Extended (Journaled), the size must be at lease 16GB and the name is optional, i choose InstallationMedia

Mount DMG file

In Disk Utility select File, Open Disk Image. Select the path to your DMG file, it can be in Downloads Folder or your external drive.

Now open the terminal

In Recovery it is available in Tools, terminal after closing the Disk Utility. You need to run createinstallmedia app from /Volumes/macOS\ %VERSION_NAME%/Install\ macOS\ %VERSION_NAME%.app/Contents/Resources with vloume you created

For example:

/Volumes/macOS\ 10.13.6\ High\ Sierra/Install\ macOS\ 10.13.6\ High\ Sierra/.app/Contents/Resources/createinstallmedia --volume /Volumes/InstallationMedia

This will create a boot media on your internal hard drive

Install it

Just reboot your mac with [Alt/Option] button pressed and select installation drive