First download the install app bundle. Older versions can be hard to find but the current version of the time you read this, can be found in the App Store.
Create and attach a drive wtih hdiutil
To create a drive run this command:
hdiutil create -o /tmp/Mojave.cdr -size 6200m -layout SPUD -fs HFS+J.
Next you will need to mount that drive
hdiutil attach /tmp/Mojave.cdr.dmg -noverify -mountpoint /Volumes/install_build.
Create installation media in the new drive
Use the downloaded app bundle to create the media:
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build.