How. do I create a bootable Flash Drive using an older version of MacOS than what is currently installed on my iMac?

I have a 32Bit DOS Emulator that will not work in 64Bit Catalina OS, and I'd like to create a bootable FlashDrive using Mojave. When I try to download Mojave from the App Store, it downloads the 6GB file then tells me that I have a newer version installed and doesn't save the download, wasting my time. Any suggestions?