I was tasked with breaking the user registration form of a Drupal 8 installation into a multi-step form. This was necessary because depending on what a user selected in step one, they were shown different form fields in step two.
To begin, I added all of the fields I needed to the user profile using the GUI. (/admin/config/people/accounts/fields). Then, I added the fields I needed to the manage form display/register tab (/admin/config/people/accounts/form-display/register).