How do I set field mapping in the Feeds module for Drupal 8?
First, select a target to map to.

This will result into a list of sources to appear. Select one of these sources.



Notes:


The UI for CSV parser is still incomplete. See https://www.drupal.org/node/2669974
If the sources list does not appear after selecting a target, you may be facing javascript issues. In this case, check with a debugger which scripts fails. In D7, the jQuery Update module (or how that modules was configured) sometimes was the culprit.
If there are no targets appearing, try to rebuild caches.
Currently there is a bug with updating items if the target 'Feeds item' is not in the list. Be sure to map to that target if you not only want to insert new items, but also update existing ones. See https://www.drupal.org/node/2857692.