I want to use iCloud for only certain files. Ho...

How can I use iCloud only for selected files?

Turn on Apple menu > System Preferences > iCloud > iCloud Drive but do not turn on Desktop and Document in the Options next to that setting. Then save the files that you want to iCloud Drive. To make it easier to find iCloud Drive you can add iCloud Drive to your Finder sidebar via Finder > Preferences > Sidebar.