Without sounding like im being sarcastic it goes on the same way it comes off 
I had a st220 on the st24 uim for a while with no issue
I had a st220 on the st24 uim for a while with no issue
Sam, if you're going to do what I think you're going to do, start the car and let it get up to temperature (and therefore a natural idle in closed loop). Just in case.
As for the TB gasket, I highly recommend one of these - though check the compatibility/aperature first. I've only used them on ST220s, and I'm not sure if the slots/vents on the ST220 are the same.
Just had a quick look thru my own collection to see if I have something. Several of these have the 'dumbell' shaped bit of plastic intact, but they are all 'lefthanded' versions off autos or other related vehicles I think. I don't think that part will have changed though. If you'd like me to post you one, I could do that in the morning - please PM me.
However, rather than waiting for a replacement, could you not just draw around it and fashion something out of a random piece of plastic after drilling a couple of holes ?