Here you can find complete guide how to install Mac OS Catalina and How Do I Create a bootable USB for Mac OS Catalina?
The easiest way is to update macOS Catalina to the new version through the Mac App Store. But if the old system had a problem, you may need a clean installation, which can not do without a bootable flash drive. It can be done in two ways: through the “Terminal” and with the help of a special utility. The first is suitable for advanced users, the second – for beginners and those who for some reason do not like to mess with the command line.
– Download the macOS Catalina files. You can download it from Mac App Store.
– Once you have your Catalina file all ready to go, close the installer that automatically pops up. You don’t want to be installing it just yet.
– Open a Finder window and head to Applications. Right-click on Install macOS 10.15 and select the Show Package Contents option.
– Open Contents > Resources. You’re going to need something from this sub-folder from here in a minute.
– Fire up Terminal either via Spotlight or from Applications > Utilities. Once open, type sudo followed by a space.
– Drag the createinstallmedia to that newly opened Terminal window.
– Now enter the following command, which will once again be followed by a space: –volume.
– Next up and back in Finder, select Go > Go To Folder. In here type /Volumes and click the Go button.
– Connect your USB drive and watch for it appearing in the Finder window.
– Drag the USB drive volume into the still open Terminal window.
– Press return, submitting the command that we just built via the magic of drag and drop.
– Type a y when prompted and hit return again.
– Go and make a coffee – this whole process can take around 30 minutes and will see the USB drive be erased before the installer is copied over.
Now that you have a bootable USB flash drive, you can install macOS Catalina from it on any compatible Mac. You need to connect the drive to your computer, restart it with the Option key pressed, then select an item from the list of available boot disks and follow the prompts of the installation wizard.