railsにdeviseを導入しました。
導入手順を残しておこうと思います。

環境

Rails	5.2.0
ruby	2.5.1
mysql	5.7.20

インストール

Gemfile

gem 'devise'

$ bundle install
$ bin/rails g devise:install
      create  config/initializers/devise.rb
      create  config/locales/devise.en.yml
===============================================================================

Some setup you must do manually if you haven't yet:

  1. Ensure you have defined default url options in your environments files. Here
     is an example of default_url_options appropriate for a development environment
     in config/environments/development.rb:

       config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }

     In production, :host should be set to the actual host of your application.

  2. Ensure you have defined root_url to *something* in your config/routes.rb.
     For example:

       root to: "home#index"

  3. Ensure you have flash messages in app/views/layouts/application.html.erb.
     For example:

       <p class="notice"><%= notice %></p>
       <p class="alert"><%= alert %></p>

  4. You can copy Devise views (for customization) to your app by running:

       rails g devise:views

===============================================================================

development.rbの編集

1の通りにconfig/environments/development.rbに以下を追加

config/environments/development.rb

config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }

rootページの作成&設定

2についてはrootのページ(http://localhost:3000) を作成します。Ruby on Rails チュートリアルの
「第3章ほぼ静的なページの作成」に習って作成したいと思います。
コンソール上では例としてroot to: "home#index"とありますが、
今回はroot to: "static_pages#home"にしたいと思います。

$ bin/rails g controller StaticPages home help
      create  app/controllers/static_pages_controller.rb
      route  get 'static_pages/home'
             get 'static_pages/help'
      invoke  erb
      create    app/views/static_pages
      create    app/views/static_pages/home.html.erb
      create    app/views/static_pages/help.html.erb
      invoke  rspec
      create    spec/controllers/static_pages_controller_spec.rb
      invoke  helper
      create    app/helpers/static_pages_helper.rb
      invoke    rspec
      invoke  assets
      invoke    coffee
      create      app/assets/javascripts/static_pages.coffee
      invoke    scss
      create      app/assets/stylesheets/static_pages.scss

config/routes.rb

root to: 'static_pages#home'

フラッシュメッセージの表示

3についてはフラッシュメッセージが表示されるようにapplication.html.erbを編集します。

apps/views/layouts/application.html.erb

<!DOCTYPE html>
<html>
  <head>
    <title>Blog1</title>
    <%= csrf_meta_tags %>
    <%= csp_meta_tag %>

    <%= stylesheet_link_tag    'application', media: 'all', 'data-turbolinks-track': 'reload' %>
    <%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
  </head>

  <body>
    <p class="notice"><%= notice %></p>
    <p class="alert"><%= alert %></p>

    <%= yield %>
  </body>
</html>

DeviseのViewファイルの作成

4はDeviseのviewファイルを作成します。

$ bin/rails g devise:views
      invoke  Devise::Generators::SharedViewsGenerator
      create    app/views/devise/shared
      create    app/views/devise/shared/_links.html.erb
      invoke  form_for
      create    app/views/devise/confirmations
      create    app/views/devise/confirmations/new.html.erb
      create    app/views/devise/passwords
      create    app/views/devise/passwords/edit.html.erb
      create    app/views/devise/passwords/new.html.erb
      create    app/views/devise/registrations
      create    app/views/devise/registrations/edit.html.erb
      create    app/views/devise/registrations/new.html.erb
      create    app/views/devise/sessions
      create    app/views/devise/sessions/new.html.erb
      create    app/views/devise/unlocks
      create    app/views/devise/unlocks/new.html.erb
      invoke  erb
      create    app/views/devise/mailer
      create    app/views/devise/mailer/confirmation_instructions.html.erb
      create    app/views/devise/mailer/email_changed.html.erb
      create    app/views/devise/mailer/password_change.html.erb
      create    app/views/devise/mailer/reset_password_instructions.html.erb
      create    app/views/devise/mailer/unlock_instructions.html.erb

管理者モデル（AdminUser）の作成

$ bin/rails g devise AdminUser
      invoke  active_record
      create    db/migrate/20180603052357_devise_create_admin_users.rb
      create    app/models/admin_user.rb
      invoke    rspec
      create      spec/models/admin_user_spec.rb
      invoke      factory_bot
      create        spec/factories/admin_users.rb
      insert    app/models/admin_user.rb
       route  devise_for :admin_users

$ bin/rails db:migrate

ログイン/ログアウト等のナビゲーションを追加

apps/views/layouts/application.html.erb

<!DOCTYPE html>
<html>
  <head>
    <title>Blog1</title>
    <%= csrf_meta_tags %>
    <%= csp_meta_tag %>

    <%= stylesheet_link_tag    'application', media: 'all', 'data-turbolinks-track': 'reload' %>
    <%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
  </head>

  <body>
    <header>
       <nav>
           <% if admin_user_signed_in? %>
               <%= link_to 'プロフィール', edit_admin_user_registration_path %>
               <%= link_to 'ログアウト', destroy_admin_user_session_path, method: :delete %>
           <% else %>
               <%= link_to 'サインアップ', new_admin_user_registration_path %>
               <%= link_to 'ログイン', new_admin_user_session_path %>
           <% end %>
       </nav>
   </header>
    <p class="notice"><%= notice %></p>
    <p class="alert"><%= alert %></p>

    <%= yield %>
  </body>
</html>

確認

参考