I have pretty much made a standalone installer for each of the most recent iterations of macOS going back to El Capitan. I create them on partitions on my Tech Drive for use in troubleshooting and rebuilding peoples Macs, especially older ones. The Mojave OS is no exception here, I made an installer on one of my Tech Drive partitions. Here is one way of doing that.
There are different ways to do this, but I prefer to use a neat little free App called Install Disk Creator. It will work for macOS High Sierra and Mojave. I highly recommend this little App, it works great and makes creating an installer seamless. However, before you can use this App you need to have the installer of the OS, in this case Mojave, on your Mac.
Open the Mac App Store and click on the Mojave install image. It brings up this download screen:
Click on “Download” and the Mojave Installer is downloaded to your Applications Folder.
Once it is finished downloading, either one of two things will happen. It will open automatically ready for install. In this case, just quit the installer at this point. It will remain in your Applications Folder until installed. Sometimes the installer does not automatically startup. In this case you will see the word “Open” in place of the word “Download” in the App Store install screen. In this case just quit the App Store and move on to the Install Disk Creator App.
Once Install Disk Creator is open you need to do three things:
First, click on the drop down at the top of the window and select the volume (Flash Drive or whatever) that is going to be created into the installer. It should be plugged into your Mac already. Once that volume is selected then click on the “Choose a macOS installer” button right below it. Navigate to wherever the Mojave installer is located on your Mac (probably the Applications Folder) and select it. Then all you have to do is click “Create Installer.” Install Disk Creator creates the installer on the media you have supplied:
It gives you a warning that it is going to erase and name the media and then installs. This takes a few minutes, but when it is finished you have a Mojave installer ready to go.
When my install was finished, I did an Option ⌥ key boot (Restart, hold down Option key after chime until you see all bootable devices) and the Mojave Installer appeared. From there all you have to do is click on it which boots into the installer ready for the Install process.
This is no big deal I guess, but if you tech support machines a separate OS installer is very handy. It might even be helpful to the regular Mac user for his or her own personal troubleshooting purposes.