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To create a bootable USB stick for Mojave start with a USB stick 16GBs or larger.
Name the USB stick MyVolume.
Install macOS Mojave.app should be in your Applications folder.
Open the Terminal app (Applications/ Utilities)
Copy and paste in this command,
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
press Return
enter your Password
press Return
follow the prompt and type in Y
press Return
the creation process should start
Having created your bootable USB installer plug it into the mac you want to install Mojave on.
Plug it directly into the mac, do not plug it in to any USB hubs.
Restart the mac while pressing and holding down the option/ alt key ⌥
give it about two minutes and the Startup Manager should appear,
select the USB (It should now be called Install Mojave) and press Return.
The mac will Restart and boot to the USB where you will see a Utilities panel
select Install OS press Continue, the installation process should start follow the
prompts.
Confirm that you followed all these procedures to the letter or try again
and report back.