set_csrf_cookie.rb

class SomeController < ApplicationController
  skip_before_action :verify_authenticity_token
  before_action :set_csrf_cookie, only: :your_get_method

  def your_get_method
    render json: {
      'csrf_token' => cookies['X-CSRF-Token'],
      ...
    }
  end

  def your_post_method
    return unless cookies['X-CSRF-Token'] == request.headers['X-CSRF-Token']
    ... rest of the method ...
  end

  private

  def set_csrf_cookie
    cookies['X-CSRF-Token'] = form_authenticity_token
  end
end