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First, they mentioned "clean installation," which is different from an upgrade. So they probably want to start fresh, not keep their existing macOS setup. That means they might need to create a bootable USB drive using the DMG file or the Install macOS app from the App Store.
Also, the DMG file is the disk image used to create the bootable USB. So the user might actually need instructions on making a bootable USB from the DMG or from the Install app.
Also, check if the user is on a Mac that supports Catalina. Macs older than 2015-2016 might not support Catalina. They need to verify their Mac's compatibility. Also, the DMG file is the disk image
Wait, but if they already have the DMG, they can use Disk Utility to mount it, then run the installer. But if they need the DMG itself, they might need to find a way to get it via the App Store first. So maybe the user is confused about the process.
But wait, Apple doesn't provide direct DMG links anymore for macOS versions. They usually distribute through the App Store. For Catalina, you have to download the installer from there. However, sometimes users have issues downloading through the App Store, maybe due to network errors or app store changes. They might be looking for alternative ways. Macs older than 2015-2016 might not support Catalina
Apple's download site for macOS allows selecting older versions, but you need to go through their support page. For example, https://www.apple.com/macos/catalina/ then check eligibility and download. But sometimes that's not working if the app is no longer available in the App Store. Apple tends to remove older macOS versions from the App Store after newer ones are released.
So putting it all together: the user needs a way to get macOS Catalina DMG for a clean install. The correct method is to download it from the App Store if available. If that's not working, maybe they can use the -seed option or a Developer account, but that's complicated. Alternatively, they might need to use a friend's computer to download the installer and transfer it via a USB drive. Also, the DMG is part of the process to create the bootable USB, so explaining the steps from getting the installer to creating the USB would be helpful. but that's probably not relevant here.
If the user can't download it from the Mac App Store, they might need to use the "Install macOS Catalina" app via a different method. Some sources suggest using the -seed or developer account options, but that's probably not relevant here. Alternatively, there are third-party tools like UTM or others that bundle macOS ISOs, but I should caution against piracy or illegal downloads.