Matabele,
When you lower the barrel into the liquid nitrogen, you get a whistling jet of white nitrogen gas spewing from the muzzle. That is because the warm barrel is changing the liquid into a gaseous state at a very high rate. You need to wait 5 or 10 minutes until this ceases. That tells you that the shank of the barrel is at or near the temperature of the liquid nitrogen. Wearing heavy gloves, remove the barrel from the thermos, drop it over the threaded rod and ensure that the shank drops down into the monoblock. Drop a washer over the top, thread a nut down TIGHTLY, then wait for the temps to normalize. That's it!