DropBox to iCloud

I don't know if it's the best way but what I personally would do would be install Dropbox on my Mac and then use Finder to move the files from the DropBox folder on the Mac to the iCloud Drive folder on the Mac. Or use an internet browser and log into dropbox.com and download the files to the iCloud Drive folder on your Mac.

The iCloud Drive folder can be accessed in the Finder sidebar or by using the Finder "Go" menu.

The following may also help:

Set up iCloud on your Mac - Apple Support

Work with folders and files in iCloud Drive - Apple Support

Also I would recommend turning on iCloud Drive but not turning on the Desktop and Documents option of iCloud Drive until you clearly understand that option. See: Add your Desktop and Documents files to iCloud Drive - Apple Support