Once the patching tool has downloaded, open the DMG file to mount it.

In the virtual drive, right click the "macOS Catalina Patcher", and click "Open".

If a warning appears saying that Apple cannot check the application for malicious software, click "Open" if it is an option, if "Close" is the only option, click it, then go into Security & Privacy in System Preferences, unlock the General tab and click "Open Anyway".