To understand about n-tier, read about 3 tier here and see how it works:
Create a class library - BusinessLogic (2nd layer)
Create a class library - DataAccess (3rd layer)
Now, from your UI (1st layer), use the object model and pass on to BusinessLogic project class. This class is a Business logic class. Do the changes as per your need here.
Now, pass on the changed data from business logic class to dataAccess project class. In this class, use ADO.NET and pass on the needed values to Stored Procedure.
For getting back data, it will be transferred from DA to BL and then BL to UI layer.
Have a look at these, explaination with samples:
3-tier architecture in C#
3-Tier Architecture Examples
3-Tier Architecture in ASP.NET with C#
Now you can put as many layers in between UI & DB layer to increase it from 3-tier to n-tier as per your need.