Hi Beergander.
Its very easy to do and takes a few minutes.
Go through South Security in Terminal 5, once through all the fun stuff turn right, walk down a bit and on the left you will see the signs for the tax/vat claim office, just in there is where you can get registered for iris, show them your passport and they then do what they need to and tell you how it works.
Used it for the first time coming back from JFK the other month and it was easy and faster then going through normal passport control.
Have a good weekend
