When you set up Devise, you tell it which model it's working on (e.g. User); many/most of its methods then apply to that class. So that's where you'll want to override stuff.
Here's a comment from the Devise code at lib/devise/models/authenticatable.rb
that describes almost exactly what you want to do, if I am reading correctly.
# This is an internal method called every time Devise needs
# to send a notification/mail. This can be overriden if you
# need to customize the e-mail delivery logic. For instance,
# if you are using a queue to deliver e-mails (delayed job,
# sidekiq, resque, etc), you must add the delivery to the queue
# just after the transaction was committed. To achieve this,
# you can override send_devise_notification to store the
# deliveries until the after_commit callback is triggered:
#
# class User
# devise :database_authenticatable, :confirmable
#
# after_commit :send_pending_notifications
#
# protected
#
# def send_devise_notification(notification)
# pending_notifications << notification
# end
#
# def send_pending_notifications
# pending_notifications.each do |n|
# devise_mailer.send(n, self).deliver
# end
# end
#
# def pending_notifications
# @pending_notifications ||= []
# end
# end
#
def send_devise_notification(notification)
devise_mailer.send(notification, self).deliver
end