Insert the pipe from the back of the fuselage. Put the bellmouth in position. Bolt the bellmouth to the pipe by putting the nut on the outside and screw head on the inside. You can add two metal sheet screws placed at 120 degrees if you wish to increase the fastening effect on the bellmouth.
Have a look here on page 25.

http://www.geohei.lu/olin/data/model...%20rev%202.pdf