How do I enable :confirmable in Devise?

to “enable” confirmable, you just need to add it to your model, e.g.:

class User
  # ...
  devise :confirmable , ....
  # ...
end

after that, you’ll have to create and run a migration which adds the required columns to your model:

# rails g migration add_confirmable_to_devise
class AddConfirmableToDevise < ActiveRecord::Migration
  def self.up
    add_column :users, :confirmation_token, :string
    add_column :users, :confirmed_at,       :datetime
    add_column :users, :confirmation_sent_at , :datetime
    add_column :users, :unconfirmed_email, :string

    add_index  :users, :confirmation_token, :unique => true
  end
  def self.down
    remove_index  :users, :confirmation_token

    remove_column :users, :unconfirmed_email
    remove_column :users, :confirmation_sent_at
    remove_column :users, :confirmed_at
    remove_column :users, :confirmation_token
  end
end

see:
Adding confirmable module to an existing site using Devise

I’d recommend to check the source code to see how Confirmable works:

https://github.com/plataformatec/devise/blob/master/lib/devise/models/confirmable.rb

You could also check the RailsCast on Devise:

http://railscasts.com/episodes/209-introducing-devise

Next it would be best to search for example applications on GitHub