To make a bootable copy of Catalina on a USB drive, make sure it has at least 12 GB of free memory. Since your USB drive will be formatted in the process, it’s also important to back up the data you store on it. If your USB drive is ready, follow these steps to create a bootable copy of Catalina:
Download the Catalina installer
Connect your USB drive
Open Disk Utility and select your USB drive
To format the drive, click the Erase button
Select the OS X Extended (Journaled) format and the GUID Partition Map scheme
Open the installer and agree to the Terms and Conditions
Click Show All Disks and select the USB drive as the installation destination
Click Continue and wait for installation completion