I always used to use the selfssl tool from the iis resource kit

but since moving to windows 7 (or possibly any operating system using iis6), you don't need to anymore.

open iis console

click the computer name (top of the tree)

select 'server certificates'

and then just choose the option of 'create self-signed certificate'

once the cert is made, you will need to go into the website and pick the certificate using the 'bindings' option.

So.. that's finally one good thing about the new IIS console....