app/views/admin/sessions/new.html.erb
<%= form_tag admin_session_path, class: "form-signin" do %>
<h2 class="form-signin-heading">Please sign in</h2>
<%= text_field_tag :username, params[:username], class: "form-control", placeholder: "Username", autofocus: true, required: true %>
<%= password_field_tag :password, params[:password], class: "form-control", placeholder: "Password", required: true %>
<div class="checkbox">
<label>
<input type="checkbox" value="remember-me"> Remember me
</label>
</div>
<%= submit_tag "Sign in", class: "btn btn-lg btn-primary btn-block" %>
<% end %>