Open Disk Utility , select your USB drive, and click Erase . Name: MyVolume Format: Mac OS Extended (Journaled) Scheme: GUID Partition Map .

If you want to perform a clean install, you'll need a USB drive (at least 16GB). Plug in your USB and rename it to MyVolume .

If the hash doesn’t match, delete the file immediately and download again from a trusted source.

(if you've previously downloaded it):

Here are the steps to download the macOS Catalina 10.15.7 .dmg file: