First make normal rails app with devise user model

• $ rails new awesome-app This will create new app

• $ cd awesome-app It will change the directory to our new project

Setup Gemfile

• Add gem 'devise' and gem 'sendgrid-ruby' to the Gemfile

• finally run $ bundle install on terminal

Setup devise

• $ rails generate devise:install it will create config/initializers/devise.rb and config/locales/devise.en.yml file and follow the instruction generated by above command

• Now generate devise user model with $ rails generate devise User

• then go to app/models/user.rb and make this file look like this:
  

  class User < ApplicationRecord
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
   devise :database_authenticatable, :registerable,
          :recoverable, :rememberable, :validatable, :confirmable
  

• Uncomment the Confirmable section of migration file located on the app/db/migrate
  

  class DeviseCreateUsers < ActiveRecord::Migration[7.0]
    def change
      create_table :users do |t|
        ## Database authenticatable
        t.string :email,              null: false, default: ""
        t.string :encrypted_password, null: false, default: ""
  
        ## Recoverable
        t.string   :reset_password_token
        t.datetime :reset_password_sent_at
  
        ## Rememberable
        t.datetime :remember_created_at
  
        ## Trackable
        # t.integer  :sign_in_count, default: 0, null: false
        # t.datetime :current_sign_in_at
        # t.datetime :last_sign_in_at
        # t.string   :current_sign_in_ip
        # t.string   :last_sign_in_ip
  
        # Confirmable
        t.string   :confirmation_token
        t.datetime :confirmed_at
        t.datetime :confirmation_sent_at
        t.string   :unconfirmed_email # Only if using reconfirmable
  
        ## Lockable
        # t.integer  :failed_attempts, default: 0, null: false # Only if lock strategy is :failed_attempts
        # t.string   :unlock_token # Only if unlock strategy is :email or :both
        # t.datetime :locked_at
  
        t.timestamps null: false
      end
  
      add_index :users, :email,                unique: true
      add_index :users, :reset_password_token, unique: true
      add_index :users, :confirmation_token,   unique: true
      # add_index :users, :unlock_token,         unique: true
    end
  end
  

• Then run $ rails db:migrate

Set up SendGrid

• Create account on SendGrid

• Set up a sender identity on SendGrid by clicking this link: https://app.sendgrid.com/settings/sender_auth/senders

• Create your API key by following this link: https://app.sendgrid.com/settings/api_keys

• add gem 'dotenv-rails' on development and test group on gem file and run $ bundle install

• create .env file in your project root and add following line inside it SENDGRID_API_KEY = 'YOUR API KEY'

• Then, modify this Devise config to use your SendGrid sender identity:
  

# config/initializers/devise.rb
config.mailer_sender = 'please-change-me-at-config-initializers-devise@example.com'

Set up Action Mailer

• create config/initializers/action_mailer.rb file and add following code

 # config/initializers/action_mailer.rb
ActionMailer::Base.smtp_settings = {
    user_name: "apikey",
    password: ENV['SENDGRID_API_KEY'],
    domain: "your-domain.com",
    address: "smtp.sendgrid.net",
    port: 587,
    authentication: :plain,
    enable_starttls_auto: true
}

Now let Test it

• run the server on local $ rails server

• go to link http://localhost:3000/users/sign_up on browser

• After signing up we will get the confirmation email.