how do I copy an excel file on my mac to a flash drive

Plug the Flash Drive in. It should "mount" on yoru desktop. If not, open a Finder window and in the sidebar, click {user]'s Mac. You should see it in the list of devices:

Double click it, or click the sidebar icon if you have one.

Open a second window and browse to the Excel file.

Drag the Excel file to the Flash Drive window. It'll copy there.