You have to make sure your .net object is COM compatible
Exposing .NET components to COM | Microsoft Docs[
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(you might have to double-check what interfaces you need for vb6 to use it properly, it may need IDispatch, I forget), then you have to register the dll which is just how COM works, it is registry-based and the registration process creates the required registry entries, and from there you should be able to add a reference to it from VB6. Obviously the target machine will also need everything available that the .net object needs, and you need to be careful about what types you are using as parameters.