How do I put horizantal white ribbon on a tapered black lamp shade?

It can certainly be done, but you will need to put some “tucks” in the top of the ribbon since the upper edge of the ribbon will have less distance to cover than the lower edge. You could do this by adding basting stitches to the top edge and pull them up to create a “gather.” Or you could glue the bottom edge of the ribbon to the lampshade and then create small folds along the top edge so it will lay flat. You could use Tacky Glue to hold the folds in place.

If you use ribbon that has some stretch to it. Stretching it while putting it on it will take form.