Firstly, in IIS or Sharepoint assign permissions.  In IIS this is the ISUR account having read access to the main website, in Sharepoint, goto Application management -> Authentication providers.  Select the Zone you want and enable ""anonymous authentication"".
 
- Secondly, in Sharepoint goto the root site and click site settings, advanced permissions, and then on the menu bar, select Anonymous Access.  If this isn't present it's because step 1 didn't apply properly!  You can also use ... _layouts/setanon.aspx to set anonymous access, although be aware that the options here will be grayed out unless step 1 is compelted.

Create an external URL Goto Sharepoint central management, select operations and the goto "alternative access mappings"
Permissions - stop inheritance of permissions To stop inheriting anonymous permissions, click site settings (whilst in the site you want to change) and then click site permissions on the left hand menu.  Then in settings (on the top menu) select stop inheriting.  From here you can remove anonymous settings etc.