devise.en.yml

    sessions:
      signed_in: 'Signed in successfully.'
      signed_in_first_time: "MY CUSTOM WELCOME GREETING. DO NOT FORGET TO READ OUR GUIDELINE!"
      signed_out: 'Signed out successfully.'

routes.rb

  #devise_for :users, :experts, "admin/administrators"
  devise_for :users, "admin/administrators"
  
  devise_for :experts, :controllers => {:sessions => 'sessions'}, :skip => [:sessions] do
    get '/experts/sign_in' => 'sessions#new', :as => :new_expert_session
    post '/experts/sign_in' => 'sessions#create', :as => :expert_session
    get '/experts/sign_out' => 'sessions#destroy', :as => :destroy_expert_session
  end

sessions_controller.rb

class SessionsController < Devise::SessionsController
  def create
     resource = warden.authenticate!(:scope => resource_name, :recall => "#{controller_path}#new")
     if is_navigational_format?
       if resource.sign_in_count == 1
         set_flash_message(:notice, :signed_in_first_time)
       else
         set_flash_message(:notice, :signed_in)
       end
     end
     sign_in(resource_name, resource)
     respond_with resource, :location => redirect_location(resource_name, resource)
   end
end