• Select Project\Properties. Open the Configuration Properties folder, select the build tab, and check "Register for COM Interop."
• Set the project's name to something meaningful. E.g. NetTools.
• Add the following in front of the Class statement to expose the class to VB 6:

      <ClassInterface(ClassInterfaceType.AutoDual)> _ 

  If you don't do this, the VB 6 program can still use the class but IntelliSense won't know anything about its properties and methods.

In the VB 6 project:

• Select Project\References. Select the DLL. E.g. NetTools.
• Use the Public classes in the DLL.

This works pretty well if you are running VB .NET and VB 6 on the same computer. If you don't build the .NET assembly on the VB 6 computer, you need to perform some extra actions to register it so VB 6 can work with it. See this article on the VB Rad Web site for more instructions and some very insightful discussion.