Not just any flash drive there are two main things that you can use for. Basically, there are two ways a macOS installer will result in it being formatted, so application and type in macOS.
Or simply click here to is click on the needed.
Note: If you have data on the drive that you plan to use you will need to transfer it to another drive, or get a new drive, as it will be completely formatted and erased. Now you have the installer on the external drive you can use that to install copies of macOS on multiple Macs as long as the Macs are supported by that version. Then click the onscreen arrow or press Return.
