Once you unbolt the two nuts in the trunk, the USM will separate from the car and drop a bit, like this:
Now the next step is to lower the jack you have on the control arm and unbolt the lower mounting bolt. Which, in this picture, would be to the left and up a bit from the jack. I believe it is an 18mm.
Here's another, showing the location of everything.
This next part is what seems really easy, but sucked when I did it.
You are supposed to just disassemble the USM from the shock and reassemble. Right. The nut on the top of the shock turns within the shock tube. What I did was use an open-ended wrench on the bolt, held the top of the shock shaft with a ViseGrip and held the USM down with a screwdriver. Luckily, I had a friend there to help.
Here's a pic of how it all panned out: