app/views/auth/sessions/new.html.erb
<div class='row'>
<div class='col-md-4 col-md-offset-4'>
<h2>Sign In</h2>
<%= form_tag auth_session_path do %>
<div class='form-group'>
<%= label_tag :email %>
<%= text_field_tag :email, nil, class: 'form-control' %>
</div>
<div class='form-group'>
<%= label_tag :password %>
<%= password_field_tag :password, nil, class: 'form-control' %>
</div>
<div class='actions'>
<%= submit_tag "Sign In", class: 'btn btn-success btn-block' %>
</div>
<% end %>
</div>
</div>