If you're using C#, look up "XML documentation" in MSDN. If you're
using VB.NET, XML documentation isn't part of the current version of
VB.NET but there are plugins available to add it.

Once you've written the XML docs, you need to create the XML file
itself when you build - for C#, this is in the project properties
dialog. You should name your documentation file the same as the
assembly, but with .xml instead of .dll as the extension. Put it in the
same directory as the assembly, and VS.NET will automatically pick up
the docs when you add a reference to the assembly, I believe.