Having both Windows and Forms authentication for site

I'm trying to have a site that will allow either external users or internal users the ability to login and authenticate.  I'm not too worried about setting up forms authentication, but trying to figure out how to also allow active directory / windows authenticating allowed for our internal users.
 
Even if I have to setup a second web project that can handle windows auth would be fine, I just need a good reference to walk through the steps in setting this up.
 
Thanks!
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