<%= form_for(@sundae) do |f| %>
  <!--
    collection_check_boxes requires 4 parameters, the last two are methods that access
    the value and text from the collection, respectively. Hence the need for
    SundaesHelper.valid_flavors!
  -->
  <%= f.collection_check_boxes(:flavor_list, valid_flavors, :first, :first) do |b| %>
    <!-- FYI: I use Bootstrap 4, so I customized how the checkboxes would render -->
    <div class="form-check">
      <label class="form-check-label"><%= b.check_box class: 'form-check-input' %> <%= b.value %></label>
    </div>
  <% end %>

  <%= f.submit %>
<% end %>