How to rotate every letter in a title

You could use jquery to split the title into letters and apply the class. That would work for any html page, regardless whether it was made by WordPress, Drupal, some dude that is stuck with Dreamweaver or whatever.

The WP way would be different. If you’re new this is fairly complex, but a good start if you’re willing to learn. The title of a WP post is generated by a function called get_the_title. This function ends with a filter, that allows you to modify the resulting title. Like this, which you would put in the functions.php file of your child theme:

add_filter ('the_title', 'wpse263471_split_title');
function wpse263471_split_title ($title) {
  ... do stuff ...
  return $title;
  }

In the place of ‘do stuff’ you would have to write php code that splits $title into letters, adds <span class=slant> around them and then glues $title together before returning it.