Lift up the tube.

Clean all the surfaces, threads and insides with a degreaser and a cloth that does not leave debris. I used medical gauze pads.

Use a silicon based lubricant (not petrol based) to lube the O-ring on the tube and the one (small one) from the bottom. There are a total of 2 O-rings in this regulator.

Use lubricant only on the O-rings and make sure it does not go or spill anywhere else. Do not spray lubricant directly as it will get vary messy and you do not want it anywhere else, besides on the rubber.