A crucial part of the assembly is attaching the mouthpiece to the neck. Notice that the end of the neck has cork around it for the mouthpiece to fit onto. It is important not to damage the cork as it is essential for keeping the joint airtight.
When attaching the mouthpiece to the cork, twist the mouthpiece gently as you push it gradually about two thirds to three quarters of the way onto the cork. When you are finished, hold the neck in front of you and make sure the flat part of the mouthpiece with the hole in it is facing downwards. This is where the reed fits on the mouthpiece. Since the reed rests against your bottom lip, it is essential that this part of the mouthpiece faces downwards. You may find that the cork is very stiff and difficult to work the mouthpiece onto. If this is the case, you will need a small amount of cork grease and the problem should disappear.