http://quickleft.com/blog/how-to-add-a-custom-model-to-your-spree-app

_partials.html.erb

<!-- ## VIEW PARTIAL FILE _sub_menu.html.erb -->
<% content_for :sub_menu do %>
  <ul id="sub_nav">
    <% # Default tab helper function always on for fancy_things sub-tab so <li> constructed manually below %>
    <li class="<%= ( params[:action] == "index" ) ? "selected" : "" %>"><%= link_to t("fancy_things") , admin_fancy_things_path %></li>
    <%= tab :new_fancy_thing , :route => :new_admin_fancy_thing , :match_path => '/fancy_things/new' , :css_class => 'last' %>
  </ul>
<% end %>

<!-- ## VIEW PARTIAL FILE _form.html.erb -->
<%= f.field_container :name do %>
  <%= f.label :name, t("name")%><br />
  <%= f.text_field :name %>
<% end %>
<%= f.field_container :description do %>
  <%= f.label :description , t("description")%><br />
  <%= f.text_field :description %>
<% end %>

controller_and_routes.rb

## CONTROLLER FILE fancy_things_controller.rb
class Admin::FancyThingsController < Admin::BaseController

  # we don't want a show method
  resource_controller :except => [ :show ]

  # redirect to the edit action after create
  create.response do |wants|
    wants.html { redirect_to edit_admin_fancy_thing_url( @fancy_thing ) }
  end

  # redirect to the edit action after update
  update.response do |wants|
    wants.html { redirect_to edit_admin_fancy_thing_url( @fancy_thing ) }
  end

end

## ROUTES FILE routes.rb
Rails.application.routes.draw do
  # ...
  namespace :admin do
    # ...
    resources :fancy_things, :except => [ :show ]
    # ...
  end
  # ...
end

custom_spree_hooks.rb

## HOOKS FILE custom_spree_hooks.rb
class CustomSpreeHooks < Spree::ThemeSupport::HookListener

  # add our navigation tab
  insert_after :admin_tabs do
    %( <%= tab :fancy_things %> )
  end

end

en.yml

## FILE en.yml
en:
  fancy_things: Fancy Things
  fancy_thing: Fancy Thing
  name: Name
  description: Description
  new_fancy_thing: New Fancy Thing
  edit_fancy_thing: Edit Fancy Thing

index.html.erb

<!-- ## VIEW FILE index.html.erb -->
<%= render :partial => 'sub_menu' %>
<h1><%= t("fancy_things")%></h1>
<table class="index">
  <tr>
    <%= hook :admin_fancy_thing_index_headers do %>
      <th><%= t("name") %></th>
      <th><%= t("description") %></th>
    <% end %>
    <th>
      <%= hook :admin_fancy_thing_index_header_actions %>
    </th>
  </tr>
  <% @fancy_things.each do |fancy_thing| %>
    <tr id="<%= dom_id fancy_thing %>">
      <%- locals = { :fancy_thing => fancy_thing } %>
      <%= hook :admin_fancy_thing_index_rows, locals do %>
        <td><%= fancy_thing.name %></td>
        <td><%= fancy_thing.description %></td>
      <% end %>
      <td class="actions">
        <%= hook :admin_fancy_thing_index_row_actions, locals do %>
          <%= link_to_edit fancy_thing %>
          &nbsp;
          <%= link_to_delete fancy_thing  %>
        <% end %>
      </td>
    </tr>
  <% end %>
</table>

migration_and_model.rb

## MIGRATION FILE 20110209222632_create_fancy_things_table.rb
class CreateFancyThingsTable < ActiveRecord::Migration

  def self.up
    create_table :fancy_things do |t|
      t.string :name
      t.string :description
      t.timestamps
    end
  end

  def self.down
    drop_table :fancy_things
  end

end

## MODEL FILE fancy_thing.rb
class FancyThing < ActiveRecord::Base
  validates_presence_of :name, :description
end

resource_views.html.erb

<!-- ## VIEW FILE new.html.erb -->
<%= render :partial => 'sub_menu' %>

<h1><%=t("new_fancy_thing")%></h1>

<%= render 'shared/error_messages', :target => @fancy_thing %>

<%= form_for(:fancy_thing, :url => admin_fancy_things_url) do |f| %>
  <%= render :partial => "form", :locals => { :f => f } %>
  <%= render :partial => "admin/shared/new_resource_links" %>
<% end %>

<!-- ## VIEW FILE edit.html.erb -->
<%= render :partial => 'sub_menu' %>

<h1><%=t("edit_fancy_thing")%></h1>

<%= render 'shared/error_messages', :target => @fancy_thing %>

<%= form_for(:fancy_thing, :url => object_url, :html => { :method => :put }) do |f| %>
  <%= render :partial => "form", :locals => { :f => f } %>
  <%= render :partial => "admin/shared/edit_resource_links" %>
<% end %>