First of all, "base class" is perhaps a bit of misnomer. I'd expect to hear that used where there is an inheritance relationship, which is very different from implementing an interface, and VBA does not support inheritance.
You've created a class module that defines an interface that other modules need to implement, but that interface definition carries no implementation itself, so there should be nothing in the interface definition to refer to.
So, in your interface class, say it's named IPerson, maybe you would see a property Get and a property Let, like ....
property get FirstName as string
property let FirstName(svalue as string)
These are stubs and provide no functionality.
Now, in every module that implements that class I would expect to see something like ...
private m_fname as string
property Get IPerson_FirstName as string
IPerson_FirstName = m_fname
property Let IPerson_FirstName(svalue as string)
m_fname = svalue
... noting that all implementation exists in the class that implements the interface.
Hope this helps...