Put tape over the nut on the group head. This is to prevent scratching when you use the wrench to remove it.
To make it easier to put the new brass nut in, put tape on the 7mm socket so it holds the nut in place. Put it back inside. Hand tighten it and then use the socket wrench to tighten it all the way. Do not over-tighten because you will break the brass nut.
When tightening the sleeve back on, be careful not to hit the front panel of the machine. Do not over tighten it.