How to setup IPrincipal for a mockup?

I want to mockup IPrincipal so I did this

public Mock<IPrincipal> Principal { get; set; }

in my setup of my nunit

 Principal = new Mock<IPrincipal>();

So this should be all that I need in my nunit unit test but how about in my actual controller file?

Like how do I set it up?

For example I have a membership.Provider

So what I did was in my controller constructor I did

Provider = Membership.Provider;

So then in my controller I just used Provider.(whatever I need).

I am not sure how to setup the Principal thing in the same way.