Ensure that all the taps are closed and switch on the vacuum pump to evacuate the pre-trap tubing. Now following the video on the right; evacuate the trap and then the rest of the line by gradually opening Tap 3.
The manometer level will rise as the air is removed as shown, and the test for leaks is to observe the level as the line is isolated from the vacuum by closing Tap 3. If it holds steady then the vacuum is good, if not then a leak is present. In the latter case, try rotating the two taps and ensuring all other taps are totally closed. Re-evacuate and isolate; if there is still a leak, contact a demonstrator.
Similarly by closing Tap 2 and re-opening Tap 3 the state of the vacuum along the whole line can be examined. Once the line is leak-proof, open Tap 2 to maintain a vacuum and surround the trap with liquid nitrogen. (Take care when handling liquid nitrogen, and do not touch apparatus that has been cooled with it since skin can be cold-burned to it.)
Keep the Dewar vessel topped up during the experiment and never leave the liquid nitrogen trap open to the air for more than a few seconds lest oxygen be liquified in the trap (notable by its blue colour) creating the risk of explosion.