app/views/devise/sessions/new.html.erb
<h2>サインアップ</h2>
<%= form_for(resource, as: resource_name, url: session_path(resource_name)) do |f| %>
<div><%= f.label :username %><br />
<%= f.text_field :username, autofocus: true %></div>
<div><%= f.label :password %><br />
<%= f.password_field :password, autocomplete: "off" %></div>
<% if devise_mapping.rememberable? -%>
<div><%= f.check_box :remember_me %> <%= f.label :remember_me %></div>
<% end -%>
<div><%= f.submit "Sign in" %></div>
<% end %>
<%= render "devise/shared/links" %>