1. Create The Entity Class (e.g Entity.cs)

   Path : Nop/Core/Domain/Entity.cs

2. Create The Mapping Class (e.g EntityMap.cs)

   Path : Nop/Data/Mapping/EntityMap.cs

3. Create a Model for MVC (e.g EntityModel.cs)

   Path : Nop/Admin/Models/EntityModel.cs OR Nop/Web/Models/EntityModel.cs

4. Create a validator for model (e.g EntityValidator.cs)

   Path : Nop/Admin/Validators/EntityValidator.cs OR Nop/Web/Validators/EntityValidator.cs

5. Create A Mapping Configuration On AutoMapperStartupTask.cs for Entity and Model

   Path : Nop/Admin/Infrastructure OR Nop/Web/Infrastructure

6. Apply Mapping between Model and Entity on MappingExtensions.cs

   Path : Nop/Admin OR Nop/Web

7. Create a service class and service interface (e.g EntityService.cs , IEntityService.cs)

   Path : Nop/Services/EntityService.cs AND Nop/Services/IEntityService.cs

8. Finally Create Controller and View for given model

as Nop Commerce uses the very first release of MVC3, database migration is not supported and you must make changes to database tables by hand. Because MVC code-first must drop and recreate your database for reflecting changes to your database.