如何回顾性地为.NET应用程序创建单元测试？

I am new to unit testing but am beginning to think I need it. I have an ASP.NET web forms application that is being extended in unforeseen directions meaning that certain bits of code are being adapted for multiple uses. I need to try and ensure that when changing these units I don't break their original intended use. So how do I best go about applying unit tests retrospectively to existing code? Practical advice appreciated. ThanksI am new to unit testing but am beginning to th